INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 months	$ 1,148	$ 4,574
12 months or more	1,100	4,447
Total	2,248	9,021
Gross Unrealized Losses
Less than 12 months	(3)	(76)
12 months or more	(5)	(137)
Total	(8)	(213)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	48	179
12 months or more	23	153
Total	71	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	315	956
12 months or more	204	1,019
Total	519	1,975
Gross Unrealized Losses
Less than 12 months	(1)	(16)
12 months or more	(2)	(38)
Total	(3)	(54)
Tax-exempt municipal securities
Fair Value
Less than 12 months	58	809
12 months or more	75	1,648
Total	133	2,457
Gross Unrealized Losses
Less than 12 months	0	(9)
12 months or more	0	(28)
Total	0	(37)
Residential mortgage-backed securities
Fair Value
Less than 12 months		0
12 months or more		15
Total		15
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	118	372
12 months or more	36	133
Total	154	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	20	824
12 months or more	607	40
Total	627	864
Gross Unrealized Losses
Less than 12 months	0	(7)
12 months or more	(3)	0
Total	(3)	(7)
Corporate debt securities
Fair Value
Less than 12 months	589	1,434
12 months or more	155	1,439
Total	744	2,873
Gross Unrealized Losses
Less than 12 months	(2)	(35)
12 months or more	0	(63)
Total	$ (2)	$ (98)