Securities - Carrying Amount and Fair Value of Securities Held to Maturity by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities held to maturity
Within One Year, Carrying Amount	$ 300
Within One Year, Fair Value	299
Over One Year Through Five Years, Carrying Amount	1,079
Over One Year Through Five Years, Fair Value	1,085
Over Five Years Through Ten Years, Carrying Amount	830
Over Five Years Through Ten Years, Fair Value	806
Over Ten Years, Carrying Amount	8,788
Over Ten Years, Fair Value	8,539
Total securities held to maturity, carrying amount	10,997	$ 9,885
Securities held to maturity	10,729	9,799
U.S. Treasury
Securities held to maturity
Within One Year, Carrying Amount	251
Within One Year, Fair Value	250
Over One Year Through Five Years, Carrying Amount	276
Over One Year Through Five Years, Fair Value	276
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
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	830
Over Five Years Through Ten Years, Fair Value	806
Over Ten Years, Carrying Amount	8,141
Over Ten Years, Fair Value	7,904
Total securities held to maturity, carrying amount	8,971	7,841
Securities held to maturity	8,710	$ 7,726
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Within One Year, Carrying Amount	49
Within One Year, Fair Value	49
Over One Year Through Five Years, Carrying Amount	803
Over One Year Through Five Years, Fair Value	809
Over Five Years Through Ten Years, Carrying Amount	0
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Carrying Amount	647
Over Ten Years, Fair Value	635
Total securities held to maturity, carrying amount	1,499
Securities held to maturity	$ 1,493

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
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-Name Accounting Standards Codification
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Fair value of held-to-maturity debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of held-to-maturity debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of held-to-maturity debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of held-to-maturity debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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