INVESTMENT SECURITIES (AFS Debt Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 11,810,001	$ 14,347,346
Gross Unrealized Gains	240,894	42,380
Gross Unrealized Loss	(2,972)	(49,968)
Fair Value	12,047,923	14,339,758
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,505,434	4,086,733
Gross Unrealized Gains	16,362	4,497
Gross Unrealized Loss	(1)	(292)
Fair Value	1,521,795	4,090,938
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	98,389	139,696
Gross Unrealized Gains	110	39
Gross Unrealized Loss	(13)	(22)
Fair Value	98,486	139,713
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	113,611	138,839
Gross Unrealized Gains	983	1,034
Gross Unrealized Loss	(1,329)	(1,473)
Fair Value	113,265	138,400
State and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5	9
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	5	9
GNMA - Residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,154,798	4,868,512
Gross Unrealized Gains	111,781	12,895
Gross Unrealized Loss	(161)	(16,066)
Fair Value	4,266,418	4,865,341
GNMA - Commercial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,002,210	773,889
Gross Unrealized Gains	26,394	6,954
Gross Unrealized Loss	0	(1,785)
Fair Value	1,028,604	779,058
FHLMC and FNMA - Residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,866,733	4,270,426
Gross Unrealized Gains	78,804	14,296
Gross Unrealized Loss	(1,465)	(30,325)
Fair Value	4,944,072	4,254,397
FHLMC and FNMA - Commercial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	68,821	69,242
Gross Unrealized Gains	6,460	2,665
Gross Unrealized Loss	(3)	(5)
Fair Value	$ 75,278	$ 71,902