Adjustments To Additional Paid In Capital, Contribution Of Subsidiary From Shareholder

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Servicing Fee Income, Related Party

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RELATED PARTY TRANSACTIONS (Narrative 10-Q) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Contribution from shareholder		$ 41,570,000	$ 0	$ 75,901,000
Interest-bearing deposits with an affiliate bank	$ 750,000,000		750,000,000		$ 0
BSI \| Affiliated Entity
Related Party Transaction [Line Items]
Interest-bearing deposits with an affiliate bank	1,000,000,000.0		1,000,000,000.0
Santander \| SC \| Affiliated Entity
Related Party Transaction [Line Items]
Servicing fee income	5,200,000	7,700,000	11,200,000	16,100,000
Collections due to Santander	7,100,000		7,100,000		$ 8,200,000
Purchase of Retail Installment Contracts \| SC \| Affiliated Entity
Related Party Transaction [Line Items]
Purchases of RICs	$ 1,700,000,000	$ 1,900,000,000	$ 2,800,000,000	$ 3,000,000,000.0