INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value
Less than 12 months	$ 1,057	$ 1,148
12 months or more	576	1,100
Total	1,633	2,248
Gross Unrealized Losses
Less than 12 months	(4)	(3)
12 months or more	(2)	(5)
Total	(6)	(8)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	225	48
12 months or more	0	23
Total	225	71
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	0
Total	(1)	0
Mortgage-backed securities
Fair Value
Less than 12 months	199	315
12 months or more	0	204
Total	199	519
Gross Unrealized Losses
Less than 12 months	(1)	(1)
12 months or more	0	(2)
Total	(1)	(3)
Tax-exempt municipal securities
Fair Value
Less than 12 months	16	58
12 months or more	19	75
Total	35	133
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Residential mortgage-backed securities
Fair Value
Less than 12 months	17
12 months or more	0
Total	17
Gross Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	0
Commercial mortgage-backed securities
Fair Value
Less than 12 months	193	118
12 months or more	43	36
Total	236	154
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	0
Total	(1)	0
Asset-backed securities
Fair Value
Less than 12 months	65	20
12 months or more	498	607
Total	563	627
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	(2)	(3)
Total	(2)	(3)
Corporate debt securities
Fair Value
Less than 12 months	342	589
12 months or more	16	155
Total	358	744
Gross Unrealized Losses
Less than 12 months	(1)	(2)
12 months or more	0	0
Total	$ (1)	$ (2)