Commodity Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 MMBtu
Settlements to be paid in subsequent period
Derivative Instruments Related to Oil and Gas Production [Line Items]
Fair Value Asset	$ 2,086
Fair Value Liability	0
Natural Gas | Group Four
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Instrument Type	Swaps
Average Volume Per Day	20,000
Weighted Average Price Floor/Swap	5.31
Fair Value Asset	4,594
Fair Value Liability	0
Natural Gas | Group Five
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Instrument Type	Swaps
Average Volume Per Day	10,000
Weighted Average Price Floor/Swap	5.10
Fair Value Asset	1,824
Fair Value Liability	0
Crude Oil | Group Seven
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Instrument Type	Collars
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	90.00
Weighted Average Price Ceiling	97.00
Fair Value Asset	519
Fair Value Liability	0
Crude Oil | Group Eight
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Instrument Type	Collars
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	90.00
Weighted Average Price Ceiling	97.00
Fair Value Asset	512
Fair Value Liability	0
Crude Oil | Group Nine
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2013
Instrument Type	Collars
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	90.00
Weighted Average Price Ceiling	100.00
Fair Value Asset	518
Fair Value Liability	0
Crude Oil | Group Ten
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2013
Instrument Type	Collars
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	90.00
Weighted Average Price Ceiling	100.00
Fair Value Asset	491
Fair Value Liability	0
Crude Oil | Group Eleven
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2013
Instrument Type	Collars
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	90.00
Weighted Average Price Ceiling	100.00
Fair Value Asset	503
Fair Value Liability	0
Crude Oil | Group Twelve
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2013
Instrument Type	Collars
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	90.00
Weighted Average Price Ceiling	100.00
Fair Value Asset	521
Fair Value Liability	0
Crude Oil | Group Fourteen
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2012
Instrument Type	Swaps
Average Volume Per Day	3,000
Weighted Average Price Floor/Swap	104.40
Fair Value Asset	5,204
Fair Value Liability	0
Crude Oil | Group Fifteen
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2012
Instrument Type	Swaps
Average Volume Per Day	3,000
Weighted Average Price Floor/Swap	104.40
Fair Value Asset	4,817
Fair Value Liability	0
Crude Oil | Group Sixteen
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2013
Instrument Type	Swaps
Average Volume Per Day	2,250
Weighted Average Price Floor/Swap	103.51
Fair Value Asset	3,108
Fair Value Liability	0
Crude Oil | Group Seventeen
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2013
Instrument Type	Swaps
Average Volume Per Day	2,250
Weighted Average Price Floor/Swap	103.51
Fair Value Asset	3,004
Fair Value Liability	0
Crude Oil | Group Eighteen
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2013
Instrument Type	Swaps
Average Volume Per Day	1,500
Weighted Average Price Floor/Swap	102.77
Fair Value Asset	1,916
Fair Value Liability	0
Crude Oil | Group Nineteen
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2013
Instrument Type	Swaps
Average Volume Per Day	1,500
Weighted Average Price Floor/Swap	102.77
Fair Value Asset	1,940
Fair Value Liability	0
Crude Oil | Group Twenty
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2014
Instrument Type	Swaps
Average Volume Per Day	2,000
Weighted Average Price Floor/Swap	100.44
Fair Value Asset	2,166
Fair Value Liability	0
Crude Oil | Group Twenty One
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2014
Instrument Type	Swaps
Average Volume Per Day	2,000
Weighted Average Price Floor/Swap	100.44
Fair Value Asset	2,232
Fair Value Liability	0
Crude Oil | Group Twenty Two
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2014
Instrument Type	Swaps
Average Volume Per Day	1,500
Weighted Average Price Floor/Swap	100.20
Fair Value Asset	1,687
Fair Value Liability	0
Crude Oil | Group Twenty Three
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2014
Instrument Type	Swaps
Average Volume Per Day	1,500
Weighted Average Price Floor/Swap	100.20
Fair Value Asset	1,699
Fair Value Liability	0
Crude Oil | Group Twenty Four
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2013
Instrument Type	Swaption
Average Volume Per Day	1,100
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	290
Crude Oil | Group Twenty Five
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2013
Instrument Type	Swaption
Average Volume Per Day	1,000
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	241
Crude Oil | Group Twenty Six
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2013
Instrument Type	Swaption
Average Volume Per Day	900
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	180
Crude Oil | Group Twenty Seven
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2013
Instrument Type	Swaption
Average Volume Per Day	750
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	117
Crude Oil | Group Twenty Eight
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	First quarter 2014
Instrument Type	Swaption
Average Volume Per Day	812
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	338
Crude Oil | Group Twenty Nine
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Second quarter 2014
Instrument Type	Swaption
Average Volume Per Day	812
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	338
Crude Oil | Group Thirty
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Third quarter 2014
Instrument Type	Swaption
Average Volume Per Day	812
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	339
Crude Oil | Group Thirty One
Derivative Instruments Related to Oil and Gas Production [Line Items]
Term	Fourth quarter 2014
Instrument Type	Swaption
Average Volume Per Day	812
Weighted Average Price Floor/Swap	100.00
Fair Value Asset	0
Fair Value Liability	$ 339