Securities - Carrying Amount and Fair Value of Securities Held to Maturity by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Securities held to maturity
Within One Year, Carrying Amount	$ 0
Within One Year, Fair Value	0
Over One Year Through Five Years, Carrying Amount	645
Over One Year Through Five Years, Fair Value	672
Over Five Years Through Ten Years, Carrying Amount	1,171
Over Five Years Through Ten Years, Fair Value	1,223
Over Ten Years, Carrying Amount	5,827
Over Ten Years, Fair Value	6,005
Total securities held to maturity, carrying amount	7,643	$ 9,421
Securities held to maturity	7,900	9,508
U.S. Treasury and government agencies
Securities held to maturity
Within One Year, Carrying Amount	0
Within One Year, Fair Value	0
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	441
Over Five Years Through Ten Years, Fair Value	459
Over Ten Years, Carrying Amount	1,294
Over Ten Years, Fair Value	1,286
Total securities held to maturity, carrying amount	1,735	1,359
Securities held to maturity	1,745	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	645
Over One Year Through Five Years, Fair Value	672
Over Five Years Through Ten Years, Carrying Amount	730
Over Five Years Through Ten Years, Fair Value	764
Over Ten Years, Carrying Amount	4,533
Over Ten Years, Fair Value	4,719
Total securities held to maturity, carrying amount	5,908	8,062
Securities held to maturity	$ 6,155	$ 8,155

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

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