Derivative Instruments - Gross Amounts of Recognized Derivative Assets and Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross amounts of recognized assets	$ 72,275	$ 84,415
Derivative Asset, Fair Value, Gross Asset	(46)	0
Net amounts of assets on balance sheet	72,229	84,415
Gross amounts of recognized liabilities	(1,167)	(4,770)
Derivative Liability, Fair Value, Gross Asset	0	16
Net amounts of liabilities on balance sheet	$ (1,167)	$ (4,754)