Securities - Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 708	$ 6,455
Less than 12 months Unrealized Losses	(8)	(70)
12 months or more Fair Value	200	15,111
12 months or more Unrealized Losses	(4)	(394)
Total Fair Value	908	21,566
Total Unrealized Losses	(12)	(464)
U.S. Treasury and federal agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		0
Less than 12 months Unrealized Losses		0
12 months or more Fair Value		97
12 months or more Unrealized Losses		(1)
Total Fair Value		97
Total Unrealized Losses	0	(1)
Agency mortgage-backed securities | Residential mortgage backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	121	3,235
Less than 12 months Unrealized Losses	(1)	(21)
12 months or more Fair Value	4	7,892
12 months or more Unrealized Losses	0	(221)
Total Fair Value	125	11,127
Total Unrealized Losses	(1)	(242)
Agency mortgage-backed securities | Commercial mortgage backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	134	2,022
Less than 12 months Unrealized Losses	(1)	(37)
12 months or more Fair Value	0	5,260
12 months or more Unrealized Losses	0	(127)
Total Fair Value	134	7,282
Total Unrealized Losses	(1)	(164)
Non-agency mortgage-backed securities | Commercial mortgage backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		884
Less than 12 months Unrealized Losses		(6)
12 months or more Fair Value		1,621
12 months or more Unrealized Losses		(41)
Total Fair Value		2,505
Total Unrealized Losses	0	(47)
Asset-backed securities and other debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	453	314
Less than 12 months Unrealized Losses	(6)	(6)
12 months or more Fair Value	196	241
12 months or more Unrealized Losses	(4)	(4)
Total Fair Value	649	555
Total Unrealized Losses	(10)	(10)
Obligations of states and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Total Unrealized Losses	$ 0	$ 0