Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013 Institution Lenders	Mar. 31, 2012
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ (593,000)
Unrealized pre-tax derivative gain recorded in AOCI	91,900,000
Gains (Losses) in oil and gas sales related to settled hedging transactions	36,500,000	57,600,000
Ineffective unrealized and realized gains (losses) recognized income	(2,891,000)	237,000
Accumulated other comprehensive income, unrealized net gains	103,600,000
Accumulated other comprehensive income, unrealized net gains, after tax	63,200,000
Number of financial institutions with whom we conduct derivative activities	15
Number of secured lenders not in banking credit facility	2
Unrealized net gains during 2013
Derivative [Line Items]
Unrealized net gains, reclassify into earnings	2,300,000
Swap Oil
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	4,500,000
Average price of SWAP per MCF	94.62
Collars Gas
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	258,600,000
Average floor price	4.08
Average cap price	4.64
Collar Oil
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	1,600,000
Average floor price	88.23
Average cap price	100.00
2013
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings	80,900,000
2014
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings	$ 10,900,000
Natural gas
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	77,700,000
Average price of SWAP per MCF	3.71
Swap NGLs (Natural Gasoline)
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	1,800,000
Average price of SWAP per MCF	92.72
Ngls C3 Component | Collars Gas
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	2,300,000
Average price of SWAP per MCF	37.00