INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value
Less than 12 months	$ 389	$ 4,574
12 months or more	1,986	4,447
Total	2,375	9,021
Gross Unrealized Losses
Less than 12 months	(2)	(76)
12 months or more	(15)	(137)
Total	(17)	(213)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	34	179
12 months or more	71	153
Total	105	332
Gross Unrealized Losses
Less than 12 months	0	(1)
12 months or more	0	(2)
Total	0	(3)
Mortgage-backed securities
Fair Value
Less than 12 months	38	956
12 months or more	546	1,019
Total	584	1,975
Gross Unrealized Losses
Less than 12 months	0	(16)
12 months or more	(8)	(38)
Total	(8)	(54)
Tax-exempt municipal securities
Fair Value
Less than 12 months	0	809
12 months or more	294	1,648
Total	294	2,457
Gross Unrealized Losses
Less than 12 months	0	(9)
12 months or more	(1)	(28)
Total	(1)	(37)
Residential mortgage-backed securities
Fair Value
Less than 12 months	0	0
12 months or more	1	15
Total	1	15
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Commercial mortgage-backed securities
Fair Value
Less than 12 months	0	372
12 months or more	70	133
Total	70	505
Gross Unrealized Losses
Less than 12 months	0	(8)
12 months or more	0	(6)
Total	0	(14)
Asset-backed securities
Fair Value
Less than 12 months	308	824
12 months or more	452	40
Total	760	864
Gross Unrealized Losses
Less than 12 months	(1)	(7)
12 months or more	(2)	0
Total	(3)	(7)
Corporate debt securities
Fair Value
Less than 12 months	9	1,434
12 months or more	552	1,439
Total	561	2,873
Gross Unrealized Losses
Less than 12 months	(1)	(35)
12 months or more	(4)	(63)
Total	$ (5)	$ (98)