|**BASIS OF PRESENTATION AND ACCOUNTING POLICIES (Impact on Balance Sheet) (Details) - USD ($) $ in Thousands**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | Sep. 30, 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | Jun. 30, 2018                                                                                                                              | Mar. 31, 2018 | Jan. 01, 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  | Dec. 31, 2017 | Sep. 30, 2017                                                                                                                                                                                                                                                                      | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------ | ------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------- | ------------- | -------------
|**New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Assets - LHFI                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | $ 85,061,913                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | [1],[2]                                                                                                                                    | $ 83,275,441  | $ 80,213,783                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | $ 80,796,195  | $ 80,790,681                                                                                                                                                                                                                                                                       | [1],[2]       | $ 81,570,388  | $ 82,964,451 
|Other assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 3,900,905                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | [2],[3]                                                                                                                                    | 3,953,909     | 4,014,012                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | 3,642,813     | 3,646,405                                                                                                                                                                                                                                                                          | [2],[3]       | 3,880,797     | 4,094,296    
|Total assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 131,628,075                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                            | 130,122,747   | 129,211,443                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 128,276,447   | 128,274,525                                                                                                                                                                                                                                                                        |               | 131,966,650   | 134,737,553  
|Liabilities - other liabilities                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 1,093,672                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | [2]                                                                                                                                        |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | 798,025       | 799,403                                                                                                                                                                                                                                                                            | [2]           |               |              
|Total liabilities                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | 107,897,176                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                            | 105,982,366   | 105,380,845                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | 104,582,315   | 104,583,693                                                                                                                                                                                                                                                                        |               | 108,508,982   | 111,851,612  
|Stockholders' equity - retained earnings                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | 3,758,505                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                            | 3,924,734     | 3,677,969                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | 3,457,257     | 3,453,957                                                                                                                                                                                                                                                                          |               | 3,361,173     | 3,308,521    
|Total stockholders' equity                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | 23,730,899                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                            | $ 24,140,381  | $ 23,830,598                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | $ 23,694,132  | $ 23,690,832                                                                                                                                                                                                                                                                       |               | $ 23,457,668  | $ 22,885,940 
|Calculated under Revenue Guidance in Effect before Topic 606                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|**New Accounting Pronouncements or Change in Accounting Principle [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Assets - LHFI                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 85,057,325                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Other assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 3,904,173                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Total assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | 131,626,755                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Liabilities - other liabilities                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 1,095,050                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Total liabilities                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | 107,898,554                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Stockholders' equity - retained earnings                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | 3,755,807                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
|Total stockholders' equity                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | $ 23,728,201                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                            |               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |                                                                                                                                                                                                                                                                                    |               |               |              
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|[1]
LHFI includes $136.5 million and $186.5 million of loans recorded at fair value at September 30, 2018 and December 31, 2017, respectively.[2]
The Company has interests in certain securitization trusts ("Trusts") that are considered variable interest entities ("VIEs") for accounting purposes. At September 30, 2018 and December 31, 2017, LHFI included $23.9 billion and $22.7 billion, Operating leases assets, net included $13.2 billion and $10.2 billion, restricted cash included $1.6 billion and $2.0 billion, other assets included $721.8 million and $747.1 million, Borrowings and other debt obligations included $31.2 billion and $28.5 billion, and Other Liabilities included $113.2 million and $198.0 million of assets or liabilities that were included within VIEs, respectively. See Note 6 to these Condensed Consolidated Financial Statements for additional information.[3]
Includes mortgage servicing rights ("MSRs") of $160.1 million and $146.0 million at September 30, 2018 and December 31, 2017, respectively, for which the Company has elected the FVO. See Note 8 to these Condensed Consolidated Financial Statements for additional information. | [1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | LHFI includes $136.5 million and $186.5 million of loans recorded at fair value at September 30, 2018 and December 31, 2017, respectively. | [2]           | The Company has interests in certain securitization trusts ("Trusts") that are considered variable interest entities ("VIEs") for accounting purposes. At September 30, 2018 and December 31, 2017, LHFI included $23.9 billion and $22.7 billion, Operating leases assets, net included $13.2 billion and $10.2 billion, restricted cash included $1.6 billion and $2.0 billion, other assets included $721.8 million and $747.1 million, Borrowings and other debt obligations included $31.2 billion and $28.5 billion, and Other Liabilities included $113.2 million and $198.0 million of assets or liabilities that were included within VIEs, respectively. See Note 6 to these Condensed Consolidated Financial Statements for additional information. | [3]           | Includes mortgage servicing rights ("MSRs") of $160.1 million and $146.0 million at September 30, 2018 and December 31, 2017, respectively, for which the Company has elected the FVO. See Note 8 to these Condensed Consolidated Financial Statements for additional information.
|[1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | LHFI includes $136.5 million and $186.5 million of loans recorded at fair value at September 30, 2018 and December 31, 2017, respectively.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|[2]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | The Company has interests in certain securitization trusts ("Trusts") that are considered variable interest entities ("VIEs") for accounting purposes. At September 30, 2018 and December 31, 2017, LHFI included $23.9 billion and $22.7 billion, Operating leases assets, net included $13.2 billion and $10.2 billion, restricted cash included $1.6 billion and $2.0 billion, other assets included $721.8 million and $747.1 million, Borrowings and other debt obligations included $31.2 billion and $28.5 billion, and Other Liabilities included $113.2 million and $198.0 million of assets or liabilities that were included within VIEs, respectively. See Note 6 to these Condensed Consolidated Financial Statements for additional information.
|[3]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | Includes mortgage servicing rights ("MSRs") of $160.1 million and $146.0 million at September 30, 2018 and December 31, 2017, respectively, for which the Company has elected the FVO. See Note 8 to these Condensed Consolidated Financial Statements for additional information.                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

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