Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Companies	Sep. 30, 2011
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings			$ 110,300,000
Gains (Losses) in oil and gas sales related to settled hedging transactions	61,500,000	26,800,000	197,700,000	80,700,000
Ineffective unrealized and realized gains (losses) recognized income	(3,719,000)	(1,935,000)	(1,610,000)	7,089,000
Number of financial institutions with whom we conduct derivative activities			16
Number of counter parties not included in bank credit facility			2
Swap Oil [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	3,300,000		3,300,000
Average price of SWAP per MCF	95.70		95.70
Collars Gas [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	232.0		232.0
Average floor price	4.23		4.23
Average cap price	4.83		4.83
Collar Oil [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	2,000,000		2,000,000
Average floor price	86.88		86.88
Average cap price	98.17		98.17
Call Option Oil [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	400,000		400,000
Weighted average price	85.00		85.00
Put Option Oil [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	200,000		200,000
Weighted average price	80.00		80.00
Derivative assets liabilities at fair value net	139,300,000		139,300,000
2012 [Member]
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings			43,400,000
2013 [Member]
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings			71,800,000
2014 [Member]
Derivative [Line Items]
Unrealized pre-tax derivative gain expected to be reclassified into earnings			$ (4,900,000)
Natural gas [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	89.6		89.6
Average price of SWAP per MCF	3.62		3.62
NGL [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	3,000,000		3,000,000
Average price of SWAP per MCF	95.80		95.80
NGLs (C3Component) [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivatives	2,400,000		2,400,000
Average price of SWAP per MCF	36.27		36.27