INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value
Less than 12 months	$ 5,450	$ 1,057
12 months or more	313	576
Total	5,763	1,633
Gross Unrealized Losses
Less than 12 months	(164)	(4)
12 months or more	(1)	(2)
Total	(165)	(6)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	401	225
12 months or more	4	0
Total	405	225
Gross Unrealized Losses
Less than 12 months	(11)	(1)
12 months or more	0	0
Total	(11)	(1)
Mortgage-backed securities
Fair Value
Less than 12 months	2,588	199
12 months or more	11	0
Total	2,599	199
Gross Unrealized Losses
Less than 12 months	(57)	(1)
12 months or more	0	0
Total	(57)	(1)
Tax-exempt municipal securities
Fair Value
Less than 12 months	77	16
12 months or more	5	19
Total	82	35
Gross Unrealized Losses
Less than 12 months	(2)	0
12 months or more	0	0
Total	(2)	0
Residential
Fair Value
Less than 12 months	86	17
12 months or more	0	0
Total	86	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	362	193
12 months or more	55	43
Total	417	236
Gross Unrealized Losses
Less than 12 months	(12)	(1)
12 months or more	0	0
Total	(12)	(1)
Asset-backed securities
Fair Value
Less than 12 months	112	65
12 months or more	193	498
Total	305	563
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	(2)
Total	(1)	(2)
Corporate debt securities
Fair Value
Less than 12 months	1,824	342
12 months or more	45	16
Total	1,869	358
Gross Unrealized Losses
Less than 12 months	(80)	(1)
12 months or more	(1)	0
Total	$ (81)	$ (1)