Securities - Securities Held to Maturity with a Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Securities held to maturity
Less than 12 months, Fair Value	$ 4,024	$ 4,603
Held To Maturity Securities Continuous Unrealized Loss Position Less than Twelve Months Aggregate Losses Recognized in Other Comprehensive Income	0	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	61	93
12 months or more, Fair Value	3,225	2,834
Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses Recognized in Other Comprehensive Income	89	104
12 months or more, Unrealized Losses, Not Recognized in OCI	23	10
Total, Fair Value	7,249	7,437
Held To Maturity Securities Unrealized Holding Losses Recognized in Other Comprehensive Income	89	104
Total, Unrealized Losses, Not Recognized in OCI	84	103
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities held to maturity
Less than 12 months, Fair Value	4,024	4,492
Held To Maturity Securities Continuous Unrealized Loss Position Less than Twelve Months Aggregate Losses Recognized in Other Comprehensive Income	0	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	61	92
12 months or more, Fair Value	1,710	1,386
Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses Recognized in Other Comprehensive Income	34	41
12 months or more, Unrealized Losses, Not Recognized in OCI	17	4
Total, Fair Value	5,734	5,878
Held To Maturity Securities Unrealized Holding Losses Recognized in Other Comprehensive Income	34	41
Total, Unrealized Losses, Not Recognized in OCI	78	96
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Less than 12 months, Fair Value	0	111
Held To Maturity Securities Continuous Unrealized Loss Position Less than Twelve Months Aggregate Losses Recognized in Other Comprehensive Income	0	0
Less than 12 months, Unrealized Losses, Not Recognized in OCI	0	1
12 months or more, Fair Value	1,515	1,448
Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses Recognized in Other Comprehensive Income	55	63
12 months or more, Unrealized Losses, Not Recognized in OCI	6	6
Total, Fair Value	1,515	1,559
Held To Maturity Securities Unrealized Holding Losses Recognized in Other Comprehensive Income	55	63
Total, Unrealized Losses, Not Recognized in OCI	$ 6	$ 7