Securities - Carrying Amount and Fair Value of Securities Held to Maturity by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Securities held to maturity
Within One Year, Carrying Amount	$ 566
Within One Year, Fair Value	565
Over One Year Through Five Years, Carrying Amount	803
Over One Year Through Five Years, Fair Value	806
Over Five Years Through Ten Years, Carrying Amount	1,830
Over Five Years Through Ten Years, Fair Value	1,793
Over Ten Years, Carrying Amount	8,121
Over Ten Years, Fair Value	7,808
Total securities held to maturity, carrying amount	11,320	$ 9,885
Securities held to maturity	10,972	9,799
U.S. Treasury
Securities held to maturity
Within One Year, Carrying Amount	518
Within One Year, Fair Value	516
Over One Year Through Five Years, Carrying Amount	9
Over One Year Through Five Years, Fair Value	10
Over Five Years Through Ten Years, Carrying Amount	0
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Carrying Amount	0
Over Ten Years, Fair Value	0
Total securities held to maturity, carrying amount	527	525
Securities held to maturity	526	527
US Government Agencies Debt Securities [Member]
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	772
Over Five Years Through Ten Years, Fair Value	771
Over Ten Years, Carrying Amount	0
Over Ten Years, Fair Value	0
Total securities held to maturity, carrying amount	772
Securities held to maturity	771
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	0
Over One Year Through Five Years, Fair Value	0
Over Five Years Through Ten Years, Carrying Amount	1,058
Over Five Years Through Ten Years, Fair Value	1,022
Over Ten Years, Carrying Amount	7,494
Over Ten Years, Fair Value	7,196
Total securities held to maturity, carrying amount	8,552	7,841
Securities held to maturity	8,218	$ 7,726
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Within One Year, Carrying Amount	48
Within One Year, Fair Value	49
Over One Year Through Five Years, Carrying Amount	794
Over One Year Through Five Years, Fair Value	796
Over Five Years Through Ten Years, Carrying Amount	0
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Carrying Amount	627
Over Ten Years, Fair Value	612
Total securities held to maturity, carrying amount	1,469
Securities held to maturity	$ 1,457

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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