INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value
Less than 12 months	$ 1,816	$ 1,148
12 months or more	683	1,100
Total	2,499	2,248
Gross Unrealized Losses
Less than 12 months	(8)	(3)
12 months or more	(6)	(5)
Total	(14)	(8)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	662	48
12 months or more	0	23
Total	662	71
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	0	0
Total	(1)	0
Mortgage-backed securities
Fair Value
Less than 12 months	450	315
12 months or more	0	204
Total	450	519
Gross Unrealized Losses
Less than 12 months	(2)	(1)
12 months or more	0	(2)
Total	(2)	(3)
Tax-exempt municipal securities
Fair Value
Less than 12 months	127	58
12 months or more	12	75
Total	139	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	181	118
12 months or more	36	36
Total	217	154
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	(1)	0
Total	(1)	0
Asset-backed securities
Fair Value
Less than 12 months	121	20
12 months or more	537	607
Total	658	627
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	(4)	(3)
Total	(5)	(3)
Corporate debt securities
Fair Value
Less than 12 months	275	589
12 months or more	98	155
Total	373	744
Gross Unrealized Losses
Less than 12 months	(1)	(2)
12 months or more	(1)	0
Total	$ (2)	$ (2)