INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value
Less than 12 months	$ 5,466	$ 1,057
12 months or more	1,038	576
Total	6,504	1,633
Gross Unrealized Losses
Less than 12 months	(95)	(4)
12 months or more	(18)	(2)
Total	(113)	(6)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	261	225
12 months or more	181	0
Total	442	225
Gross Unrealized Losses
Less than 12 months	(4)	(1)
12 months or more	(3)	0
Total	(7)	(1)
Mortgage-backed securities
Fair Value
Less than 12 months	2,555	199
12 months or more	231	0
Total	2,786	199
Gross Unrealized Losses
Less than 12 months	(41)	(1)
12 months or more	(5)	0
Total	(46)	(1)
Tax-exempt municipal securities
Fair Value
Less than 12 months	77	16
12 months or more	28	19
Total	105	35
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	(1)	0
Total	(2)	0
Residential
Fair Value
Less than 12 months	348	17
12 months or more	0	0
Total	348	17
Gross Unrealized Losses
Less than 12 months	(3)	0
12 months or more	0	0
Total	(3)	0
Commercial
Fair Value
Less than 12 months	340	193
12 months or more	98	43
Total	438	236
Gross Unrealized Losses
Less than 12 months	(6)	(1)
12 months or more	(1)	0
Total	(7)	(1)
Asset-backed securities
Fair Value
Less than 12 months	296	65
12 months or more	277	498
Total	573	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	1,589	342
12 months or more	223	16
Total	1,812	358
Gross Unrealized Losses
Less than 12 months	(39)	(1)
12 months or more	(8)	0
Total	$ (47)	$ (1)