INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value
Less than 12 months	$ 1,586	$ 1,148
12 months or more	1,036	1,100
Total	2,622	2,248
Gross Unrealized Losses
Less than 12 months	(12)	(3)
12 months or more	(21)	(5)
Total	(33)	(8)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	342	48
12 months or more	0	23
Total	342	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	406	315
12 months or more	0	204
Total	406	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	181	58
12 months or more	12	75
Total	193	133
Gross Unrealized Losses
Less than 12 months	(3)	0
12 months or more	0	0
Total	(3)	0
Commercial mortgage-backed securities
Fair Value
Less than 12 months	204	118
12 months or more	33	36
Total	237	154
Gross Unrealized Losses
Less than 12 months	(3)	0
12 months or more	(1)	0
Total	(4)	0
Asset-backed securities
Fair Value
Less than 12 months	152	20
12 months or more	814	607
Total	966	627
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	(16)	(3)
Total	(17)	(3)
Corporate debt securities
Fair Value
Less than 12 months	301	589
12 months or more	177	155
Total	478	744
Gross Unrealized Losses
Less than 12 months	(4)	(2)
12 months or more	(4)	0
Total	$ (8)	$ (2)