Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ 70,800	$ (9,200)	$ 83,315	$ (2,100)
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	(18,800)	(26,000)	4,100	(43,400)
Foreign currency translations	1,300	800	(2,300)	300
Income tax effect	6,900	9,400	(1,300)	15,700
Net of tax	(10,600)	(15,800)	500	(27,400)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	19,500	(6,800)	43,100	(11,300)
Other comprehensive income (loss), net of tax	(30,100)	(9,000)	(42,600)	(16,100)
Ending balance, net of tax	40,656	(18,200)	40,656	(18,200)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	30,400	(9,200)	67,200	(16,300)
Derivative expense, net		(1,500)		(1,500)
Income tax effect	(10,900)	3,900	(24,100)	6,500
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	77,900	(8,000)	86,800	(1,400)
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	(18,800)	(26,000)	4,100	(43,400)
Income tax effect	6,900	9,400	(1,300)	15,700
Net of tax	(11,900)	(16,600)	2,800	(27,700)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	19,500	(6,800)	43,100	(11,300)
Other comprehensive income (loss), net of tax	(31,400)	(9,800)	(40,300)	(16,400)
Ending balance, net of tax	46,500	(17,800)	46,500	(17,800)
Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	30,400	(9,200)	67,200	(16,300)
Derivative expense, net		(1,500)		(1,500)
Income tax effect	(10,900)	3,900	(24,100)	6,500
Foreign Currency Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(7,100)	(1,200)	(3,500)	(700)
Other comprehensive income (loss) before reclassifications:
Foreign currency translations	1,300	800	(2,300)	300
Net of tax	1,300	800	(2,300)	300
Amounts reclassified from accumulated other comprehensive income:
Other comprehensive income (loss), net of tax	1,300	800	(2,300)	300
Ending balance, net of tax	$ (5,800)	$ (400)	$ (5,800)	$ (400)