Commodity Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 MMBtu
Settlements to be paid in subsequent period
Derivative Instruments Related to Oil and Gas Production [Line Items]
Fair Value Asset	$ 886
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	1,636
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	52
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	105
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	93
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	124
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	161
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	3,211
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	1,954
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	1,912
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	1,230
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	1,294
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	1,406
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	1,540
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	1,218
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	1,274
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	299
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	267
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	203
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	133
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	386
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	386
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	386
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	$ 386