Schedule of Changes In Accumulated Other Comprehensive Income Loss (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ 10,500,000		$ 28,833,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	30,000,000		9,900,000
Foreign currency translations	(500,000)		(500,000)
Income tax effect	(10,800,000)		(3,500,000)
Net of tax	18,700,000		5,900,000
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(4,700,000)		(10,200,000)
Other comprehensive income (loss), net of tax	14,000,000		(4,300,000)
Ending balance, net of tax	24,530,000		24,530,000
Cash Flow Hedging
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	10,500,000	(200,000)	28,833,000	21,900,000
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	30,000,000	108,100,000	9,900,000	72,600,000
Income tax effect	(10,800,000)	(38,900,000)	(3,500,000)	(26,100,000)
Net of tax	19,200,000	69,200,000	6,400,000	46,500,000
Amounts reclassified from accumulated other comprehensive income:
Net of tax	(4,700,000)	(6,000,000)	(10,200,000)	(5,400,000)
Other comprehensive income (loss), net of tax	14,500,000	63,200,000	(3,800,000)	41,100,000
Ending balance, net of tax	25,000,000	63,000,000	25,000,000	63,000,000
Reclassification out of Accumulated Other Comprehensive Income
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(7,300,000)		(15,900,000)
Income tax effect	2,600,000		5,700,000
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedging
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	(7,300,000)	(9,400,000)	(15,900,000)	(8,400,000)
Income tax effect	2,600,000	3,400,000	5,700,000	3,000,000
Foreign Currency Gain (Loss)
Other comprehensive income (loss) before reclassifications:
Foreign currency translations	(500,000)		(500,000)
Net of tax	(500,000)		(500,000)
Amounts reclassified from accumulated other comprehensive income:
Other comprehensive income (loss), net of tax	(500,000)		(500,000)
Ending balance, net of tax	$ (500,000)		$ (500,000)