Securities - Carrying Amount and Fair Value Securities Held to Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	$ 591
Over Five Years Through Ten Years, Carrying Amount	915
Over Ten Years, Carrying Amount	8,831
Total securities held to maturity, carrying amount	10,337	$ 10,158
Over One Year Through Five Years, Fair Value	597
Over Five Years Through Ten Years, Fair Value	949
Over Ten Years, Fair Value	8,770
Total securities held to maturity, Fair Value	10,316	10,207
U.S. Treasury
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	492
Over Five Years Through Ten Years, Carrying Amount	0
Over Ten Years, Carrying Amount	0
Total securities held to maturity, carrying amount	492	489
Over One Year Through Five Years, Fair Value	497
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Fair Value	0
Total securities held to maturity, Fair Value	497	493
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	50
Over Five Years Through Ten Years, Carrying Amount	109
Over Ten Years, Carrying Amount	8,104
Total securities held to maturity, carrying amount	8,263	7,782
Over One Year Through Five Years, Fair Value	50
Over Five Years Through Ten Years, Fair Value	111
Over Ten Years, Fair Value	8,040
Total securities held to maturity, Fair Value	8,201	$ 7,790
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	49
Over Five Years Through Ten Years, Carrying Amount	806
Over Ten Years, Carrying Amount	727
Total securities held to maturity, carrying amount	1,582
Over One Year Through Five Years, Fair Value	50
Over Five Years Through Ten Years, Fair Value	838
Over Ten Years, Fair Value	730
Total securities held to maturity, Fair Value	$ 1,618