Securities - Carrying Amount and Fair Value Securities Held to Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	$ 796
Over Five Years Through Ten Years, Carrying Amount	1,730
Over Ten Years, Carrying Amount	7,810
Total securities held to maturity, carrying amount	10,901	$ 9,885
Over One Year Through Five Years, Fair Value	806
Over Five Years Through Ten Years, Fair Value	1,709
Over Ten Years, Fair Value	7,641
Total securities held to maturity, Fair Value	10,720	9,799
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	565
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	564
U.S. Treasury
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	9
Over Five Years Through Ten Years, Carrying Amount	0
Over Ten Years, Carrying Amount	0
Total securities held to maturity, carrying amount	528	525
Over One Year Through Five Years, Fair Value	9
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Fair Value	0
Total securities held to maturity, Fair Value	527	527
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	519
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	518
US Government Agencies Debt Securities [Member]
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	0
Over Five Years Through Ten Years, Carrying Amount	722
Over Ten Years, Carrying Amount	0
Total securities held to maturity, carrying amount	722
Over One Year Through Five Years, Fair Value	0
Over Five Years Through Ten Years, Fair Value	723
Over Ten Years, Fair Value	0
Total securities held to maturity, Fair Value	723
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	0
Over Five Years Through Ten Years, Carrying Amount	1,008
Over Ten Years, Carrying Amount	7,199
Total securities held to maturity, carrying amount	8,207	7,841
Over One Year Through Five Years, Fair Value	0
Over Five Years Through Ten Years, Fair Value	986
Over Ten Years, Fair Value	7,041
Total securities held to maturity, Fair Value	8,027	$ 7,726
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	787
Over Five Years Through Ten Years, Carrying Amount	0
Over Ten Years, Carrying Amount	611
Total securities held to maturity, carrying amount	1,444
Over One Year Through Five Years, Fair Value	797
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Fair Value	600
Total securities held to maturity, Fair Value	1,443
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	46
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 46