Securities - Carrying Amount and Fair Value of Securities Held to Maturity by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Securities held to maturity
Within One Year, Carrying Amount	$ 9
Within One Year, Fair Value	10
Over One Year Through Five Years, Carrying Amount	725
Over One Year Through Five Years, Fair Value	748
Over Five Years Through Ten Years, Carrying Amount	2,171
Over Five Years Through Ten Years, Fair Value	2,179
Over Ten Years, Carrying Amount	6,941
Over Ten Years, Fair Value	7,043
Total securities held to maturity, carrying amount	9,846	$ 10,901
Securities held to maturity	9,980	10,720
U.S. Treasury and government agencies
Securities held to maturity
Within One Year, Carrying Amount	9
Within One Year, Fair Value	10
Over One Year Through Five Years, Carrying Amount	0
Over One Year Through Five Years, Fair Value	0
Over Five Years Through Ten Years, Carrying Amount	1,239
Over Five Years Through Ten Years, Fair Value	1,241
Over Ten Years, Carrying Amount	0
Over Ten Years, Fair Value	0
Total securities held to maturity, carrying amount	1,248	1,250
Securities held to maturity	1,251	1,250
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities held to maturity
Within One Year, Carrying Amount	0
Within One Year, Fair Value	0
Over One Year Through Five Years, Carrying Amount	725
Over One Year Through Five Years, Fair Value	748
Over Five Years Through Ten Years, Carrying Amount	932
Over Five Years Through Ten Years, Fair Value	938
Over Ten Years, Carrying Amount	6,941
Over Ten Years, Fair Value	7,043
Total securities held to maturity, carrying amount	8,598	9,651
Securities held to maturity	$ 8,729	$ 9,470

Amount of investment in debt security measured at amortized cost (held-to-maturity).

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