INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value
Less than 12 months	$ 4,743	$ 1,057
12 months or more	377	576
Total	5,120	1,633
Gross Unrealized Losses
Less than 12 months	(84)	(4)
12 months or more	(3)	(2)
Total	(87)	(6)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	373	225
12 months or more	17	0
Total	390	225
Gross Unrealized Losses
Less than 12 months	(7)	(1)
12 months or more	0	0
Total	(7)	(1)
Mortgage-backed securities
Fair Value
Less than 12 months	2,336	199
12 months or more	114	0
Total	2,450	199
Gross Unrealized Losses
Less than 12 months	(39)	(1)
12 months or more	(3)	0
Total	(42)	(1)
Tax-exempt municipal securities
Fair Value
Less than 12 months	81	16
12 months or more	4	19
Total	85	35
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	0
Total	(1)	0
Residential
Fair Value
Less than 12 months	79	17
12 months or more	0	0
Total	79	17
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	0
Total	(1)	0
Commercial
Fair Value
Less than 12 months	255	193
12 months or more	74	43
Total	329	236
Gross Unrealized Losses
Less than 12 months	(5)	(1)
12 months or more	0	0
Total	(5)	(1)
Asset-backed securities
Fair Value
Less than 12 months	231	65
12 months or more	149	498
Total	380	563
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	0	(2)
Total	0	(2)
Corporate debt securities
Fair Value
Less than 12 months	1,388	342
12 months or more	19	16
Total	1,407	358
Gross Unrealized Losses
Less than 12 months	(31)	(1)
12 months or more	0	0
Total	$ (31)	$ (1)