| **VIEs - Cash Flow Summary (Details) - USD ($)** | 3 Months Ended  | 6 Months Ended  |
|--------------------------------------------------|-----------------|-----------------|
||
| VIEs, Primary Beneficiary | Trusts               |            |            |   |   |
| **Variable Interest Entity [Line Items]**        |            |            |   |   |
| Assets securitized                               | $ 3,497,142,000 | $ 7,238,608,000 | $ 8,657,428,000 | $ 11,361,659,000 |
| Net proceeds from new securitizations            | 2,091,900,000   | 5,990,485,000   | 6,141,620,000 | 9,576,609,000 |
| Net proceeds from sale of retained bonds         | 645,370,000     | 132,186,000     | 1,029,746,000 | 195,967,000 |
| Cash received for servicing fees                 | 223,836,000     | 230,408,000     | 461,429,000 | 458,595,000 |
| Net distributions from Trusts                    | 1,092,535,000   | 1,903,257,000   | 2,206,898,000 | 3,043,634,000 |
| Total cash received from Trusts                  | 4,053,641,000   | 8,256,336,000   | 9,839,693,000 | 13,274,805,000 |
| VIE, Not Primary Beneficiary                     |            |            |   |   |
| **Variable Interest Entity [Line Items]**        |            |            |   |   |
| Receivables securitized                          | 0               | 0               | 0 | 1,891,278,000 |
| Net proceeds from new securitizations            | 0               | 0               | 0 | 1,779,532,000 |
| Cash received for servicing fees                 | 5,098,000       | 9,294,000       | 11,038,000 | 16,021,000 |
| Total cash received from Trusts                  | $ 5,098,000     | $ 9,294,000     | $ 11,038,000 | $ 1,795,553,000 |

| X                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash Flows Between Transferor and Transferee, Proceeds from Sale of Retained Bonds
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Distributions Received from Variable Interest Entity
+ References
No definition available.
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| X                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Receivables Securitized
+ References
No definition available.
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| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sales of Receivables Securitization
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash Flows between a transferor and a transferee attributable to newly transferred assets related to either a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the transferred financial assets).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(1) -URI https://asc.fasb.org/extlink&oid=121570589&loc=d3e107207-111719
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash Flows between a transferee and a transferor attributable to servicing fees related to a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the transferred financial assets).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(4) -URI https://asc.fasb.org/extlink&oid=121570589&loc=d3e107207-111719
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Proceeds from securitizations of receivables treated as collateralized borrowings, which are classified as financing transactions.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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