Accumulated Other Comprehensive Income (Loss) - Schedule of Changes In Accumulated Other Comprehensive Income Loss (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ (9,200,000)	$ 10,500,000	$ (2,062,000)	$ 28,800,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	(26,000,000)	30,000,000	(43,400,000)	9,900,000
Foreign currency translations	800,000	(500,000)	300,000	(500,000)
Income tax effect	9,400,000	(10,800,000)	15,700,000	(3,500,000)
Net of tax	(15,800,000)	18,700,000	(27,400,000)	5,900,000
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(6,800,000)	4,700,000	(11,300,000)	10,200,000
Other comprehensive income (loss), net of tax	(9,000,000)	14,000,000	(16,100,000)	(4,300,000)
Ending balance, net of tax	(18,161,000)	24,500,000	(18,161,000)	24,500,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(9,200,000)	7,300,000	(16,300,000)	15,900,000
Derivative expense, net	(1,500,000)		(1,500,000)
Income tax effect	3,900,000	(2,600,000)	6,500,000	(5,700,000)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(8,000,000)	10,500,000	(1,400,000)	28,800,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	(26,000,000)	30,000,000	(43,400,000)	9,900,000
Income tax effect	9,400,000	(10,800,000)	15,700,000	(3,500,000)
Net of tax	(16,600,000)	19,200,000	(27,700,000)	6,400,000
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(6,800,000)	4,700,000	(11,300,000)	10,200,000
Other comprehensive income (loss), net of tax	(9,800,000)	14,500,000	(16,400,000)	(3,800,000)
Ending balance, net of tax	(17,800,000)	25,000,000	(17,800,000)	25,000,000
Cash Flow Hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(9,200,000)	7,300,000	(16,300,000)	15,900,000
Derivative expense, net	(1,500,000)		(1,500,000)
Income tax effect	3,900,000	(2,600,000)	6,500,000	(5,700,000)
Foreign Currency Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(1,200,000)		(700,000)
Other comprehensive income (loss) before reclassifications:
Foreign currency translations	800,000	(500,000)	300,000	(500,000)
Net of tax	800,000	(500,000)	300,000	(500,000)
Amounts reclassified from accumulated other comprehensive income:
Other comprehensive income (loss), net of tax	800,000	(500,000)	300,000	(500,000)
Ending balance, net of tax	$ (400,000)	$ (500,000)	$ (400,000)	$ (500,000)