Fair Value - Fair Value Inputs, Assets and Liabilities, Quantitative Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	96,612us-gaap_AvailableForSaleSecurities	94,913us-gaap_AvailableForSaleSecurities
Asset-backed securities, student loans
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	12,064	12,460
Asset-backed securities, credit cards
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	2,917	3,053
Asset-backed securities, other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	4,028	4,145
State and political subdivisions
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	11,049	10,820
Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, fair value disclosure	5,356	5,447
Liabilities, fair value disclosure	181	83
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Derivative instruments, liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative instruments, foreign exchange contracts, liabilities	172	74
Derivative instruments, other, liabilities	9	9
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Derivative instruments, assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative instruments, foreign exchange contracts, assets	185	81
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Asset-backed securities, student loans
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	257	259
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Asset-backed securities, other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	3,633	3,780
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| State and political subdivisions
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	37	38
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Discounted cash flows \| Derivative instruments, liabilities \| Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Participant redemptions	5.00%	5.20%
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Discounted cash flows \| Asset-backed securities, other \| Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Credit spread	0.20%	0.20%
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Discounted cash flows \| State and political subdivisions \| Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Credit spread	2.20%	2.10%
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Option model \| Derivative instruments, liabilities \| Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Volatility	11.30%	9.00%
Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Option model \| Derivative instruments, assets \| Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Volatility	10.80%	9.10%
Significant Unobservable Inputs Readily Available \| Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, fair value disclosure	276	178
Liabilities, fair value disclosure	181	83
Significant Unobservable Inputs Readily Available \| Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Derivative instruments, liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative instruments, foreign exchange contracts, liabilities	172	74
Derivative instruments, other, liabilities	9	9
Significant Unobservable Inputs Readily Available \| Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Derivative instruments, assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative instruments, foreign exchange contracts, assets	185	81
Significant Unobservable Inputs Readily Available \| Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Asset-backed securities, student loans
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	0	0
Significant Unobservable Inputs Readily Available \| Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| Asset-backed securities, other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	54	59
Significant Unobservable Inputs Readily Available \| Pricing Methods with Significant Unobservable Market Inputs (Level 3) \| State and political subdivisions
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available for sale	37	38

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Risk associated credit spread expressed as a percentage, used as an input to measure fair value

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- Definition

Fair Value Inputs, Participant Redemptions

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

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-Topic 210

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

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Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps.

+ References

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Fair value portion of liability contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward (delivery or nondelivery) contracts, and swaps entered into.

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Fair value of financial and nonfinancial obligations.

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