Accumulated Other Comprehensive Income (Loss) - Schedule of Changes In Accumulated Other Comprehensive Income Loss (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ (2,062,000)
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	(17,400,000)
Foreign currency translations	(500,000)
Income tax effect	6,300,000
Net of tax	(11,600,000)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(4,500,000)
Other comprehensive loss, net of tax	(7,100,000)
Ending balance, net of tax	(9,151,000)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(1,400,000)	28,800,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	(17,400,000)	(20,100,000)
Income tax effect	6,300,000	7,300,000
Net of tax	(11,100,000)	(12,800,000)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(4,500,000)	5,500,000
Other comprehensive loss, net of tax	(6,600,000)	(18,300,000)
Ending balance, net of tax	8,000,000	10,500,000
Foreign Currency Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(700,000)
Other comprehensive income (loss) before reclassifications:
Foreign currency translations	(500,000)
Net of tax	(500,000)
Amounts reclassified from accumulated other comprehensive income:
Other comprehensive loss, net of tax	(500,000)
Ending balance, net of tax	1,200,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(7,100,000)
Income tax effect	2,600,000
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedges [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(7,100,000)	8,500,000
Income tax effect	$ 2,600,000	$ (3,000,000)