Accumulated Other Comprehensive Loss - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax (benefit) expense included in net unrealized gains (losses), investment securities	$ (22)	$ 70	$ (85)	$ 134
Tax included in reclassification, investment securities	(3)	(7)	(4)	(9)
Tax included in amortization, investment securities	1	2	2	5
Tax (benefit) expense included in net unrealized gains (losses), fair value option liabilities	41	(6)	46	(52)
Tax (benefit) expense included in net unrealized gains (losses), cash flow hedges (less than $1 million in all periods)	2	(3)	2	(4)
Tax included in reclassification, cash flow hedges	1	1	2	2
Tax included in net unrealized gains (losses), pension and postretirement benefit liability	$ 1	$ 1	$ 1	$ 1
Unrealized Gains (Losses) on Investment Securities [Member] \| Accounting Standards Update 2016-01, Equity Investments Previously Classified as Available-for-sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption, tax					$ 2	$ 0
Unrealized Gains (Losses) on Fair Value Option Liabilities Attributable to Our Own Credit Spread [Member] \| Accounting Standards Update 2016-01, Fair Value Option [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption, tax					$ 0	$ 103

Cumulative Effect of New Accounting Principle in Period of Adoption, Tax

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Other Comprehensive Income (Loss), Amortization of Unrealized Loss on Transfers from Available-for-Sale to Held-to Maturity Securities, Tax

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Amount of tax expense (benefit) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before reclassification adjustment, of tax expense (benefit) for unrealized gain (loss) from increase (decrease) in instrument-specific credit risk on financial liability measured under fair value option.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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