Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Detail)	Sep. 30, 2012 MMBtu
Collars [Member] | Natural Gas 2012 [Member]
Derivative [Line Items]
Volume Hedged	279,641
Average floor price	4.76
Average cap price	5.22
Collars [Member] | Natural Gas 2013 [Member]
Derivative [Line Items]
Volume Hedged	240,000
Average floor price	4.73
Average cap price	5.20
Collars [Member] | Natural Gas 2014 [Member]
Derivative [Line Items]
Volume Hedged	325,000
Average floor price	3.75
Average cap price	4.47
Collars [Member] | Crude Oil 2012 [Member]
Derivative [Line Items]
Volume Hedged	2,000
Average floor price	70.00
Average cap price	80.00
Collars [Member] | Crude Oil 2013 [Member]
Derivative [Line Items]
Volume Hedged	3,000
Average floor price	90.60
Average cap price	100.00
Collars [Member] | Crude Oil 2014 [Member]
Derivative [Line Items]
Volume Hedged	2,000
Average floor price	85.55
Average cap price	100.00
Swaps [Member] | Natural Gas 2012 [Member]
Derivative [Line Items]
Volume Hedged	270,000
Average hedge price	3.77
Swaps [Member] | Natural Gas 2013 [Member]
Derivative [Line Items]
Volume Hedged	177,521
Average hedge price	3.57
Swaps [Member] | Crude Oil 2013 [Member]
Derivative [Line Items]
Volume Hedged	5,081
Average hedge price	96.59
Swaps [Member] | Crude Oil 2014 [Member]
Derivative [Line Items]
Volume Hedged	4,000
Average hedge price	94.56
Swaps [Member] | NGLs Propane 2012 [Member]
Derivative [Line Items]
Volume Hedged	6,000
Average hedge price	38.67
Swaps [Member] | NGLs Propane 2013 [Member]
Derivative [Line Items]
Volume Hedged	5,000
Average hedge price	35.55
Call options [Member] | Crude Oil 2012 [Member]
Derivative [Line Items]
Volume Hedged	4,700
Weighted average price	85
Put options [Member] | Crude Oil 2012 [Member]
Derivative [Line Items]
Volume Hedged	2,500
Weighted average price	80
Sold swaps [Member] | NGLs Natural Gasoline 2012 [Member]
Derivative [Line Items]
Volume Hedged	12,000
Average hedge price	96.28
Sold swaps [Member] | NGLs Natural Gasoline 2013 [Member]
Derivative [Line Items]
Volume Hedged	8,000
Average hedge price	89.64
Purchased swaps [Member] | NGLs Natural Gasoline 2012 [Member]
Derivative [Line Items]
Volume Hedged	5,500
Average hedge price	82.37
Purchased swaps [Member] | NGLs Natural Gasoline 2013 [Member]
Derivative [Line Items]
Volume Hedged	1,500
Average hedge price	76.30