DESCRIPTION OF BUSINESS, BASIS OF PRESENTATION AND ACCOUNTING POLICIES (Intermediate Holding Company) (Details)	5 Months Ended
	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($) branch employee	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total assets		$ 152,353,787,000	$ 149,499,477,000	$ 143,910,431,000
Total liabilities		132,127,010,000	125,100,647,000
LHFS		241,300,000	289,000,000.0
Deposits held-for-sale		4,700,000,000	0
Commercial
Error Corrections and Prior Period Adjustments Restatement [Line Items]
LHFS		$ 1,800,000,000	$ 116,300,000
Santander BanCorp
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Number of bank branches | branch		27
Number of employees | employee		1,000
Total assets		$ 6,000,000,000.0
Total liabilities		4,800,000,000
Subsidiary
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Investments available-for-sale		1,200,000,000
LHFS		2,600,000,000
Deposits held-for-sale		4,700,000,000
Deferred tax liabilities, investments		39,000,000.0
Subsidiary | Forecast
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Sale of stock, consideration received on transaction	$ 1,100,000,000
Subsidiary | Commercial
Error Corrections and Prior Period Adjustments Restatement [Line Items]
LHFS		1,600,000,000
Subsidiary | Consumer
Error Corrections and Prior Period Adjustments Restatement [Line Items]
LHFS		$ 998,600,000