|**DEPOSITS AND OTHER CUSTOMER ACCOUNTS (Tables)** | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Deposits [Abstract]**                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Summary of Deposits and Other Customer Accounts   | Deposits and other customer accounts are summarized as follows:March 31, 2020December 31, 2019(dollars in thousands)BalancePercent of total depositsBalancePercent of total depositsInterest-bearing demand deposits$10,701,83015.6%$10,301,13315.3%Non-interest-bearing demand deposits16,624,26524.2%14,922,97422.2%Savings5,696,8478.3%5,632,1648.4%Customer repurchase accounts433,1700.6%407,4770.6%Money market27,727,11240.4%26,687,67739.6%CDs7,488,27910.9%9,375,28113.9%Total deposits(1)$68,671,503100.0%$67,326,706100.0%(1)Includes foreign deposits, as defined by the FRB, of$9.9 billionand$8.9 billionatMarch 31, 2020andDecember 31, 2019, respectively.
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|(dollars in thousands)                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Interest-bearing demand deposits                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Non-interest-bearing demand deposits              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Savings                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Customer repurchase accounts                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Money market                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|CDs                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Total deposits(1)                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|(1)                                               | Includes foreign deposits, as defined by the FRB, of$9.9 billionand$8.9 billionatMarch 31, 2020andDecember 31, 2019, respectively.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

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