Hedging Positions (Detail)	Dec. 31, 2012
Natural Gas | 2013 Hedging Position One
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.000
Natural Gas | 2013 Hedging Position Two
Derivative [Line Items]
Daily Volume	10,000
Swap Price	5.270
Natural Gas | 2013 Hedging Position Three
Derivative [Line Items]
Daily Volume	10,000
Swap Price	5.320
Natural Gas | 2014 Hedging Position One
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.000
Natural Gas | 2014 Hedging Position Two
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.040
Natural Gas | 2014 Hedging Position Three
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.105
Natural Gas | 2014 Hedging Position Four
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.250
Natural Gas | 2015 Hedging Position One
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.005
Natural Gas | 2015 Hedging Position Two
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.220
Natural Gas | 2015 Hedging Position Three
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.255
Oil | 2013 Hedging Position One
Derivative [Line Items]
Daily Volume	1,000
Swap Price	92.800
Oil | 2013 Hedging Position Two
Derivative [Line Items]
Daily Volume	2,000	[1]
Swap Price	94.050
Oil | 2013 Hedging Position Three
Derivative [Line Items]
Daily Volume	1,000
Swap Price	94.450
Oil | 2013 Hedging Position Four
Derivative [Line Items]
Daily Volume	1,000
Swap Price	94.60
Oil | 2013 Hedging Position Five
Derivative [Line Items]
Daily Volume	1,000
Swap Price	97.15
Oil | 2013 Hedging Position Six
Derivative [Line Items]
Daily Volume	1,000
Swap Price	101.53
Oil | 2013 Hedging Position Seven
Derivative [Line Items]
Daily Volume	1,000
Swap Price	103.00
Oil | 2013 Hedging Position Eight
Derivative [Line Items]
Daily Volume	1,000
Swap Price	103.15
Oil | 2013 Hedging Position Nine
Derivative [Line Items]
Daily Volume	1,000
Swap Price	104.25
Oil | 2013 Hedging Position Ten
Derivative [Line Items]
Daily Volume	1,000
Swap Price	104.47
Oil | 2013 Hedging Position Eleven
Derivative [Line Items]
Daily Volume	1,000
Swap Price	104.50
Oil | 2013 Hedging Position Twelve
Derivative [Line Items]
Daily Volume	1,000	[2]
Swap Price	107.30
Oil | 2014 Hedging Position One
Derivative [Line Items]
Daily Volume	1,000
Swap Price	90.060
Oil | 2014 Hedging Position Two
Derivative [Line Items]
Daily Volume	1,000
Swap Price	93.550
Oil | 2014 Hedging Position Three
Derivative [Line Items]
Daily Volume	1,000
Swap Price	94.000
Oil | 2014 Hedging Position Four
Derivative [Line Items]
Daily Volume	1,000
Swap Price	98.000
Oil | 2014 Hedging Position Five
Derivative [Line Items]
Daily Volume	1,000
Swap Price	98.30
Oil | 2014 Hedging Position Six
Derivative [Line Items]
Daily Volume	1,000
Swap Price	99.65
Oil | 2014 Hedging Position Three
Derivative [Line Items]
Daily Volume	1,000	[2]
Swap Price	103.30
Oil | 2015 Hedging Position One
Derivative [Line Items]
Daily Volume	1,000
Swap Price	90.000

[1] January through June

[2] Brent oil contract