Hedging Positions (Detail)	Sep. 30, 2013 MMBTU
Natural Gas [Member] | 2013 Hedging Position One [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.000
Natural Gas [Member] | 2013 Hedging Position Two [Member]
Derivative [Line Items]
Daily Volume	10,000	[1]
Swap Price	4.050
Natural Gas [Member] | 2013 Hedging Position Three [Member]
Derivative [Line Items]
Daily Volume	20,000	[2]
Swap Price	4.450
Natural Gas [Member] | 2013 Hedging Position Four [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	5.270
Natural Gas [Member] | 2013 Hedging Position Five [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	5.320
Natural Gas [Member] | 2014 Hedging Position One [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.000
Natural Gas [Member] | 2014 Hedging Position Two [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.040
Natural Gas [Member] | 2014 Hedging Position Three [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.105
Natural Gas [Member] | 2014 Hedging Position Four [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.190
Natural Gas [Member] | 2014 Hedging Position Five [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.250
Natural Gas [Member] | 2014 Hedging Position Six [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.350
Natural Gas [Member] | 2015 Hedging Position One [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.005
Natural Gas [Member] | 2015 Hedging Position Two [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.220
Natural Gas [Member] | 2015 Hedging Position Three [Member]
Derivative [Line Items]
Daily Volume	10,000
Swap Price	4.255
Oil [Member] | 2013 Hedging Position One [Member]
Derivative [Line Items]
Daily Volume	2,000	[2]
Swap Price	92.350
Oil [Member] | 2013 Hedging Position Two [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	92.800
Oil [Member] | 2013 Hedging Position Three [Member]
Derivative [Line Items]
Daily Volume	2,000	[3]
Swap Price	94.050
Oil [Member] | 2013 Hedging Position Four [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	94.450
Oil [Member] | 2013 Hedging Position Five [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	94.600
Oil [Member] | 2013 Hedging Position Six [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	97.15
Oil [Member] | 2013 Hedging Position Seven [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	101.53
Oil [Member] | 2013 Hedging Position Eight [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	103.00
Oil [Member] | 2013 Hedging Position Nine [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	103.15
Oil [Member] | 2013 Hedging Position Ten [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	104.25
Oil [Member] | 2013 Hedging Position Eleven [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	104.47
Oil [Member] | 2013 Hedging Position Twelve [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	104.50
Oil [Member] | 2013 Hedging Position Thirteen [Member]
Derivative [Line Items]
Daily Volume	1,000	[4]
Swap Price	107.30
Oil [Member] | 2014 Hedging Position One [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	90.060
Oil [Member] | 2014 Hedging Position Two [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	92.250
Oil [Member] | 2014 Hedging Position Three [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	93.550
Oil [Member] | 2014 Hedging Position Four [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	94.000
Oil [Member] | 2014 Hedging Position Five [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	98.000
Oil [Member] | 2014 Hedging Position Six [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	98.300
Oil [Member] | 2014 Hedging Position Seven [Member]
Derivative [Line Items]
Daily Volume	2,000	[3]
Swap Price	98.85
Oil [Member] | 2014 Hedging Position Eight [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	99.65
Oil [Member] | 2014 Hedging Position Nine [Member]
Derivative [Line Items]
Daily Volume	1,000	[4]
Swap Price	103.30
Oil [Member] | 2015 Hedging Position One [Member]
Derivative [Line Items]
Daily Volume	1,000
Swap Price	90.000

[1] April through December

[2] July through December

[3] January through June

[4] Brent crude oil contract