BORROWINGS (Santander Bank) (Details) - SBNA - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total borrowings and other debt obligations	$ 5,400,000	$ 7,160,943
Effective Rate	1.14%	2.34%
FHLB advances, maturing through May 2022
Debt Instrument [Line Items]
FHLB advances, Balance	$ 5,400,000	$ 7,035,000
Effective Rate	1.14%	2.15%
REIT preferred, callable May 2020
Debt Instrument [Line Items]
REIT preferred, Balance	$ 0	$ 125,943
Effective Rate	0.00%	13.17%
FHLB advances
Debt Instrument [Line Items]
Letters of credit	$ 340,800

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant