Derivative Financial Instruments - Impact of Derivative Financial Instruments within Condensed Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Net settlements on matured derivatives	$ 4,760	$ (921)
Net settlements on early terminations and modifications of derivatives	11,069	0
Change in fair value of unsettled derivatives, net	(14,185)	(1,127)
Total gain (loss) on derivatives, net	$ 1,644	$ (2,048)