Derivative Instruments - Gross Amounts of Recognized Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross amounts of recognized assets	$ (89,811)	$ (84,415)
Derivative Asset, Fair Value, Gross Asset	0	0
Net amounts of assets on balance sheet	89,811	84,415
Gross amounts of recognized liabilities	(830)	(4,770)
Derivative Liability, Fair Value, Gross Asset	0	16
Net amounts of liabilities on balance sheet	$ (830)	$ (4,754)

Fair value, after offset of derivative liability, of financial asset or other 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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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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