Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative [Line Items]
Unrealized pre-tax derivative gain recorded in AOCI			$ 60,500,000
Gains (Losses) in oil and gas sales related to settled hedging transactions	30,500,000	78,600,000	67,000,000	136,200,000
Ineffective unrealized and realized gains (losses) recognized income		1,872,000	(2,891,000)	2,109,000
Accumulated other comprehensive income, unrealized net gains			103,600,000
Accumulated other comprehensive income, unrealized net gains, after tax			63,200,000
Derivative assets liabilities at fair value net	127,500,000		127,500,000
2013
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings			49,500,000
2014
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings			10,900,000
Unrealized net gains during 2013
Derivative [Line Items]
Unrealized net gains, reclassify into earnings			$ 3,200,000