Securities - Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 501	$ 6,455
Less than 12 months Unrealized Losses	(8)	(70)
12 months or more Fair Value	1,872	15,111
12 months or more Unrealized Losses	(15)	(394)
Total Fair Value	2,373	21,566
Total Unrealized Losses	(23)	(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	6	3,235
Less than 12 months Unrealized Losses	0	(21)
12 months or more Fair Value	1,427	7,892
12 months or more Unrealized Losses	(8)	(221)
Total Fair Value	1,433	11,127
Total Unrealized Losses	(8)	(242)
Agency mortgage-backed securities | Commercial mortgage backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	149	2,022
Less than 12 months Unrealized Losses	(1)	(37)
12 months or more Fair Value	259	5,260
12 months or more Unrealized Losses	(2)	(127)
Total Fair Value	408	7,282
Total Unrealized Losses	(3)	(164)
Non-agency mortgage-backed securities | Commercial mortgage backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	0	884
Less than 12 months Unrealized Losses	0	(6)
12 months or more Fair Value	5	1,621
12 months or more Unrealized Losses	0	(41)
Total Fair Value	5	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	345	314
Less than 12 months Unrealized Losses	(7)	(6)
12 months or more Fair Value	181	241
12 months or more Unrealized Losses	(5)	(4)
Total Fair Value	526	555
Total Unrealized Losses	(12)	(10)
Obligations of states and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	1
Total Unrealized Losses	$ 0	$ 0