INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value
Less than 12 months	$ 2,173	$ 1,148
12 months or more	1,909	1,100
Total	4,082	2,248
Gross Unrealized Losses
Less than 12 months	(86)	(3)
12 months or more	(85)	(5)
Total	(171)	(8)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	2	48
12 months or more	0	23
Total	2	71
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Mortgage-backed securities
Fair Value
Less than 12 months	67	315
12 months or more	1	204
Total	68	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	309	58
12 months or more	176	75
Total	485	133
Gross Unrealized Losses
Less than 12 months	(10)	0
12 months or more	(2)	0
Total	(12)	0
Commercial mortgage-backed securities
Fair Value
Less than 12 months	198	118
12 months or more	131	36
Total	329	154
Gross Unrealized Losses
Less than 12 months	(15)	0
12 months or more	(13)	0
Total	(28)	0
Asset-backed securities
Fair Value
Less than 12 months	144	20
12 months or more	891	607
Total	1,035	627
Gross Unrealized Losses
Less than 12 months	(5)	0
12 months or more	(41)	(3)
Total	(46)	(3)
Corporate debt securities
Fair Value
Less than 12 months	1,453	589
12 months or more	710	155
Total	2,163	744
Gross Unrealized Losses
Less than 12 months	(55)	(2)
12 months or more	(29)	0
Total	$ (84)	$ (2)