Investment Securities - Additional Information (Details) $ in Millions			12 Months Ended
Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 147	$ 207	$ 147	$ 207	$ 199
Number of sale securities exceeding amortized cost over fair value \| Security	920		920
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (458)	(535)	$ (458)	(535)
Difference in amortized cost and fair value of securities that had been in a loss position for 12 months or longer	$ (373)	(116)	$ (373)	(116)
Securities held to maturity \| Security	250		250
Fair value of securities pledged	$ 2,800	1,900	$ 2,800	1,900
Carrying value of securities pledged	5,700	8,100	5,700	8,100
Fair value of securities pledged, accepted	1	16	1	16
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	$ (182)	$ (199)	(182)	(199)
Investment Securities Portfolio \| US Treasury and Government
Schedule of Available-for-sale Securities [Line Items]
Percentage of portfolio	95.00%	91.00%
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (403)	$ (421)	(403)	(421)
Difference in amortized cost and fair value of securities that had been in a loss position for 12 months or longer	(336)	(90)	(336)	(90)
Non-Agency \| Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities			1	9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1)	(9)	(1)	(9)
Difference in amortized cost and fair value of securities that had been in a loss position for 12 months or longer	$ (1)	$ (7)	$ (1)	$ (7)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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