Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income Loss (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ (18,200,000)	$ 24,500,000	$ (2,062,000)	$ 28,800,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	47,100,000	(30,800,000)	3,700,000	(20,900,000)
Foreign currency translations	(1,600,000)	300,000	(1,300,000)	(200,000)
Income tax effect	(16,900,000)	11,100,000	(1,200,000)	7,600,000
Net of tax	28,600,000	(19,400,000)	1,200,000	(13,500,000)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(800,000)	(1,300,000)	(12,100,000)	8,900,000
Other comprehensive income (loss), net of tax	29,400,000	(18,100,000)	13,300,000	(22,400,000)
Ending balance, net of tax	11,189,000	6,400,000	11,189,000	6,400,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(1,300,000)	(2,100,000)	(17,600,000)	13,800,000
Derivative expense, net			(1,500,000)
Income tax effect	500,000	800,000	7,000,000	(4,900,000)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(17,800,000)	25,000,000	(1,400,000)	28,800,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	47,100,000	(30,800,000)	3,700,000	(20,900,000)
Income tax effect	(16,900,000)	11,100,000	(1,200,000)	7,600,000
Net of tax	30,200,000	(19,700,000)	2,500,000	(13,300,000)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(800,000)	(1,300,000)	(12,100,000)	8,900,000
Other comprehensive income (loss), net of tax	31,000,000	(18,400,000)	14,600,000	(22,200,000)
Ending balance, net of tax	13,200,000	6,600,000	13,200,000	6,600,000
Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(1,300,000)	(2,100,000)	(17,600,000)	13,800,000
Derivative expense, net			(1,500,000)
Income tax effect	500,000	800,000	7,000,000	(4,900,000)
Foreign Currency Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(400,000)	(500,000)	(700,000)
Other comprehensive income (loss) before reclassifications:
Foreign currency translations	(1,600,000)	300,000	(1,300,000)	(200,000)
Net of tax	(1,600,000)	300,000	(1,300,000)	(200,000)
Amounts reclassified from accumulated other comprehensive income:
Other comprehensive income (loss), net of tax	(1,600,000)	300,000	(1,300,000)	(200,000)
Ending balance, net of tax	$ (2,000,000)	$ (200,000)	$ (2,000,000)	$ (200,000)