Derivative Instruments - Reconciles Net Amounts Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative assets	$ 76,478	$ 18,389
Noncurrent derivative assets	91,566	32,231
Net amounts of assets on balance sheet	168,044	50,620
Derivative liabilities	(6,809)	(12,999)
Noncurrent derivative liabilities	0	(2,173)
Net amounts of liabilities on balance sheet	(6,809)	(15,172)
Total derivative assets, net	$ 161,235	$ 35,448