Fair Value Measurements - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
Apr. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities portfolio, value		$ 29,558,500,000us-gaap_AvailableForSaleSecurities	$ 28,392,800,000us-gaap_AvailableForSaleSecurities
Trading account securities		4,700,000us-gaap_TradingSecurities	1,700,000us-gaap_TradingSecurities
Settlement of contingent consideration	55,300,000
Transfers into Level 3 assets		0	0
Transfers out of Level 3 assets		0	0
Transfers into Level 3 liabilities		0	0
Transfers out of Level 3 liabilities		0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio		881	831
Available for sale securities portfolio, value		25,000,000,000	26,400,000,000
Loans \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value		14,200,000	34,000,000
Other Real Estate Owned \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value		4,100,000	1,400,000
External Pricing \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio		881	829
Available for sale securities portfolio, value			26,300,000,000
Trading account securities		4,700,000	1,700,000
Internal Pricing \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio			2
Available for sale securities portfolio, value			$ 57,400,000

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Fair Value, Measurement with Unobservable Inputs Reconciliation, NonRecurring Basis, Asset and Liabilities

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- Definition

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

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

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- Definition

Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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- Definition

Balance held at close of period in number of contracts.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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