Derivative Instruments - Reconciles Net Amounts Derivative Assets and Liabilities (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Oct. 31, 2014 Subsequent Event [Member]
Derivative [Line Items]
Derivative assets	$ 143,268,000	$ 3,616,000
Noncurrent derivative assets	31,002,000	0
Net amounts of assets on balance sheet	174,270,000	3,616,000	27,000,000
Derivative liabilities	0	(90,535,000)
Noncurrent derivative liabilities	0	(7,829,000)
Net amounts of liabilities on balance sheet	0	98,364,000	7,000,000
Total derivative assets (liabilities), net			$ 20,000,000