Commodity Derivative Positions (Detail) (Commodity contracts, USD $)	6 Months Ended
	Jun. 30, 2011 MMBtu
Derivative Instruments Related to Oil and Gas Production [Line Items]
Fair Value Asset	$ 14,272,000
Fair Value Liability	115,000
Natural Gas | Group one
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2011
Average Volume Per Day	30,000
Weighted Average Price Floor/Swap	4.83
Weighted Average Price Ceiling	6.00
Fair Value Asset	1,615,000
Instrument Type	Costless collars
Natural Gas | Group Two
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	6.00
Weighted Average Price Ceiling	8.50
Fair Value Asset	2,703,000
Instrument Type	Costless collars
Natural Gas | Group Three
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2012
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	6.00
Weighted Average Price Ceiling	8.50
Fair Value Asset	2,349,000
Instrument Type	Costless collars
Natural Gas | Group Four
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2011
Average Volume Per Day	40,000
Weighted Average Price Floor/Swap	5.06
Fair Value Asset	2,532,000
Instrument Type	Swaps
Natural Gas | Group Five
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.01
Fair Value Asset	399,000
Instrument Type	Swaps
Natural Gas | Group Six
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2012
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.10
Fair Value Asset	250,000
Instrument Type	Swaps
Natural Gas | Group Seven
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2012
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	5.31
Fair Value Asset	1,098,000
Instrument Type	Swaps
Natural Gas | Group Eight
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	5.31
Fair Value Asset	935,000
Instrument Type	Swaps
Natural Gas | Group Nine
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.10
Fair Value Asset	76,000
Instrument Type	Swaps
Crude Oil | Group Ten
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2011
Average Volume Per Day	360
Weighted Average Price Floor/Swap	80.00
Weighted Average Price Ceiling	103.30
Fair Value Liability	24,000
Instrument Type	Costless collars
Crude Oil | Group 11
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	360
Weighted Average Price Floor/Swap	80.00
Weighted Average Price Ceiling	103.30
Fair Value Liability	91,000
Instrument Type	Costless collars
Crude Oil | Group 12
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	317,000
Instrument Type	Costless collars
Crude Oil | Group 13
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	300,000
Instrument Type	Costless collars
Crude Oil | Group 14
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	292,000
Instrument Type	Costless collars
Crude Oil | Group 15
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Average Volume Per Day	500
Weighted Average Price Floor/Swap	100.00
Weighted Average Price Ceiling	120.00
Fair Value Asset	295,000
Instrument Type	Costless collars
Crude Oil | Group 16
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2011
Average Volume Per Day	500
Weighted Average Price Floor/Swap	109.00
Fair Value Asset	591,000
Instrument Type	Swaps
Crude Oil | Group 17
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2011
Average Volume Per Day	500
Weighted Average Price Floor/Swap	109.00
Fair Value Asset	$ 520,000
Instrument Type	Swaps