Derivative Instruments - Offsetting of Derivative Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Gross Amount	$ 46,160	$ 43,369
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities	0	0
Net Amount Presented in the Balance Sheet	46,160us-gaap_DerivativeLiabilities	43,369us-gaap_DerivativeLiabilities
Derivative Liabilities Subject to Master Netting Arrangement [Member]
Derivatives, Fair Value [Line Items]
Net Amount Presented in the Balance Sheet	45,985	40,323
Derivative Liabilities Subject to Master Netting Arrangement [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Gross Amount	45,985	40,323
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities	0	0
Net Amount Presented in the Balance Sheet	45,985	40,323
Derivative Liabilities Not Subject to Master Netting Arrangement [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Gross Amount	128	3,014
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities	0	0
Net Amount Presented in the Balance Sheet	128	3,014
Derivative Liabilities Not Subject to Master Netting Arrangement [Member] \| Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Gross Amount	47	32
Gross Amounts Offset in the Balance Sheet, Derivative Liabilities	0	0
Net Amount Presented in the Balance Sheet	$ 47	$ 32

Fair value, after offset of derivative asset, of 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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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.

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