Derivatives - Offsetting Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Gross Amounts of Recognized Assets	$ 20,390	$ 19,448
Gross Amounts Offset in the Statement of Financial Position
Net Amounts of Assets Presented in the Statement of Financial Position	20,390	19,448
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets
Net Amount, Assets	20,390	19,448
Gross Amounts of Recognized Liabilities	21,219	20,455
Gross Amounts Offset in the Statement of Financial Position
Net Amounts of Liabilities Presented in the Statement of Financial Position	21,219	20,455
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Liability
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Pledged, Liability	15,032	16,839
Net Amount, Liability	$ 6,187	$ 3,616

Fair value of liability associated with derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205G

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44C

-Subparagraph a

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

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Fair value of the gross assets less the gross liabilities of a derivative asset or group of derivative assets.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

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The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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The amount as of the balance sheet date of the fair value of derivative assets that in accordance with the entity's accounting policy was not offset against an obligation to return cash collateral under a master netting arrangement.

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

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-Publisher FASB

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair value of the gross liabilities less the gross assets of a derivative liability or group of derivative liabilities.

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