INVESTMENT SECURITIES (AFS Debt Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 10,887,310	$ 14,347,346
Gross Unrealized Gains	199,026	42,380
Gross Unrealized Loss	(7,364)	(49,968)
Fair Value	11,078,972	14,339,758
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	212,609	4,086,733
Gross Unrealized Gains	3,045	4,497
Gross Unrealized Loss	0	(292)
Fair Value	215,654	4,090,938
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	172,845	139,696
Gross Unrealized Gains	78	39
Gross Unrealized Loss	(15)	(22)
Fair Value	172,908	139,713
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	111,577	138,839
Gross Unrealized Gains	854	1,034
Gross Unrealized Loss	(1,290)	(1,473)
Fair Value	111,141	138,400
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3	9
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	3	9
GNMA - Residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,322,848	4,868,512
Gross Unrealized Gains	84,241	12,895
Gross Unrealized Loss	(194)	(16,066)
Fair Value	3,406,895	4,865,341
GNMA - Commercial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,413,584	773,889
Gross Unrealized Gains	26,142	6,954
Gross Unrealized Loss	(193)	(1,785)
Fair Value	1,439,533	779,058
FHLMC and FNMA - Residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,587,145	4,270,426
Gross Unrealized Gains	78,402	14,296
Gross Unrealized Loss	(5,670)	(30,325)
Fair Value	5,659,877	4,254,397
FHLMC and FNMA - Commercial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	66,699	69,242
Gross Unrealized Gains	6,264	2,665
Gross Unrealized Loss	(2)	(5)
Fair Value	$ 72,961	$ 71,902