Securities - Carrying Amount and Fair Value of Securities Held to Maturity by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Securities held to maturity
Within One Year, Carrying Amount	$ 4
Within One Year, Fair Value	4
Over One Year Through Five Years, Carrying Amount	687
Over One Year Through Five Years, Fair Value	714
Over Five Years Through Ten Years, Carrying Amount	2,134
Over Five Years Through Ten Years, Fair Value	2,185
Over Ten Years, Carrying Amount	6,043
Over Ten Years, Fair Value	6,282
Total securities held to maturity, carrying amount	8,868	$ 9,421
Securities held to maturity	9,185	9,508
U.S. Treasury and government agencies
Securities held to maturity
Within One Year, Carrying Amount	4
Within One Year, Fair Value	4
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	1,300
Over Five Years Through Ten Years, Fair Value	1,322
Over Ten Years, Carrying Amount	0
Over Ten Years, Fair Value	0
Total securities held to maturity, carrying amount	1,304	1,359
Securities held to maturity	1,326	1,353
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	687
Over One Year Through Five Years, Fair Value	714
Over Five Years Through Ten Years, Carrying Amount	834
Over Five Years Through Ten Years, Fair Value	863
Over Ten Years, Carrying Amount	6,043
Over Ten Years, Fair Value	6,282
Total securities held to maturity, carrying amount	7,564	8,062
Securities held to maturity	$ 7,859	$ 8,155

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

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