DESCRIPTION OF BUSINESS, BASIS OF PRESENTATION AND ACCOUNTING POLICIES (General) (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
	Sep. 01, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Aug. 31, 2020	Dec. 31, 2019
Noncontrolling Interest [Line Items]
Income tax (benefit)/provision		$ (53,343)		$ 112,927	$ (272,856)	$ 384,467
Total assets		145,739,595		147,185,414	145,739,595	147,185,414		$ 149,499,477
Total liabilities		124,934,624			124,934,624			125,100,647
Stockholder's equity		19,532,351			19,532,351			22,021,460
Net (loss)/income including NCI		874,536		270,436	(1,144,182)	916,292
Santander BanCorp
Noncontrolling Interest [Line Items]
Total assets							$ 5,500,000	6,000,000
Total liabilities							4,300,000	4,800,000
Stockholder's equity							$ 1,200,000	$ 1,200,000
Net (loss)/income including NCI		14,300		$ 26,400	33,100	$ 66,600
Disposal group, not discontinued operation, loans transferred					160,000
Disposal group, not discontinued operation, real estate owned, transferred					$ 30,000
Disposal Group, Not Discontinued Operations | Santander BanCorp
Noncontrolling Interest [Line Items]
Sale of stock, consideration received on transaction	$ 1,280,000
Proceeds from divestiture of interest in consolidated subsidiaries		62,000
Income tax (benefit)/provision		12,000	$ 39,000
Gain (loss) on disposition of business		50,000
Reclassification from accumulated other comprehensive income, current period, before tax		23,600
Reclassification from accumulated other comprehensive income, current period, net of tax		14,800
Disposal group, not discontinued operation, transaction expenses		$ 10,000
SC
Noncontrolling Interest [Line Items]
Ownership percentage by parent		80.20%			80.20%
Percentage owned by noncontrolling shareholders		19.80%			19.80%