INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 months	$ 1,177	$ 4,574
12 months or more	4,031	4,447
Total	5,208	9,021
Gross Unrealized Losses
Less than 12 months	(7)	(76)
12 months or more	(54)	(137)
Total	(61)	(213)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	25	179
12 months or more	214	153
Total	239	332
Gross Unrealized Losses
Less than 12 months	0	(1)
12 months or more	(1)	(2)
Total	(1)	(3)
U.S. government corporations and agencies, mortgage-backed securities
Fair Value
Less than 12 months	255	956
12 months or more	1,152	1,019
Total	1,407	1,975
Gross Unrealized Losses
Less than 12 months	(1)	(16)
12 months or more	(21)	(38)
Total	(22)	(54)
Tax-exempt municipal securities
Fair Value
Less than 12 months	30	809
12 months or more	924	1,648
Total	954	2,457
Gross Unrealized Losses
Less than 12 months	0	(9)
12 months or more	(5)	(28)
Total	(5)	(37)
Residential mortgage-backed securities
Fair Value
Less than 12 months	0	0
12 months or more	15	15
Total	15	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	42	372
12 months or more	170	133
Total	212	505
Gross Unrealized Losses
Less than 12 months	(1)	(8)
12 months or more	(2)	(6)
Total	(3)	(14)
Asset-backed securities
Fair Value
Less than 12 months	755	824
12 months or more	70	40
Total	825	864
Gross Unrealized Losses
Less than 12 months	(5)	(7)
12 months or more	(1)	0
Total	(6)	(7)
Corporate debt securities
Fair Value
Less than 12 months	70	1,434
12 months or more	1,486	1,439
Total	1,556	2,873
Gross Unrealized Losses
Less than 12 months	0	(35)
12 months or more	(24)	(63)
Total	$ (24)	$ (98)