Securities - Securities Held to Maturity with a Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities held to maturity
Less than 12 months, Fair Value	$ 4,382	$ 3,162
Less than 12 months, Unrealized Losses, Recognized in OCI	32	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	83	32
12 months or more, Fair Value	6,133	5,496
12 months or more, Unrealized Losses, Recognized in OCI	122	83
12 months or more, Unrealized Losses, Not Recognized in OCI	205	108
Total, Fair Value	10,515	8,658
Total, Unrealized Losses, Recognized in OCI	154	83
Total, Unrealized Losses, Not Recognized in OCI	288	140
Securities held to maturity	10,729	9,799
Held-to-maturity Securities	10,997	9,885
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	20	54
U.S. Treasury securities
Securities held to maturity
Less than 12 months, Fair Value	494	494
Less than 12 months, Unrealized Losses, Recognized in OCI	0	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	3	1
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses, Recognized in OCI	0	0
12 months or more, Unrealized Losses, Not Recognized in OCI	0	0
Total, Fair Value	494	494
Total, Unrealized Losses, Recognized in OCI	0	0
Total, Unrealized Losses, Not Recognized in OCI	3	1
Securities held to maturity	526	527
Held-to-maturity Securities	527	525
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	2	3
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities held to maturity
Less than 12 months, Fair Value	3,841	2,649
Less than 12 months, Unrealized Losses, Recognized in OCI	32	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	79	31
12 months or more, Fair Value	4,686	4,000
12 months or more, Unrealized Losses, Recognized in OCI	74	31
12 months or more, Unrealized Losses, Not Recognized in OCI	188	99
Total, Fair Value	8,527	6,649
Total, Unrealized Losses, Recognized in OCI	106	31
Total, Unrealized Losses, Not Recognized in OCI	267	130
Securities held to maturity	8,710	7,726
Held-to-maturity Securities	8,971	7,841
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	6	15
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Less than 12 months, Fair Value	47	19
Less than 12 months, Unrealized Losses, Recognized in OCI	0	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	1	0
12 months or more, Fair Value	1,447	1,496
12 months or more, Unrealized Losses, Recognized in OCI	48	52
12 months or more, Unrealized Losses, Not Recognized in OCI	17	9
Total, Fair Value	1,494	1,515
Total, Unrealized Losses, Recognized in OCI	48	52
Total, Unrealized Losses, Not Recognized in OCI	18	9
Securities held to maturity	1,493	1,546
Held-to-maturity Securities	1,499	1,519
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	$ 12	$ 36