Derivative Instruments and Hedging Activities - Hedging Positions (Detail)	Sep. 30, 2014 MBbls
Oil [Member] | 2014 Hedging Position One [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	90.06
Oil [Member] | 2014 Hedging Position Two [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	90.25
Oil [Member] | 2014 Hedging Position Three [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	92.25
Oil [Member] | 2014 Hedging Position Four [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	93.55
Oil [Member] | 2014 Hedging Position Five [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	94.00
Oil [Member] | 2014 Hedging Position Six [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	98.00
Oil [Member] | 2014 Hedging Position Seven [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	98.30
Oil [Member] | 2014 Hedging Position Eight [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	2,000
Swap Price	98.85
Oil [Member] | 2014 Hedging Position Nine [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	99.65
Oil [Member] | 2014 Hedging Position Ten [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	103.30
Oil [Member] | 2015 Hedging Position One [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	89.00
Oil [Member] | 2015 Hedging Position Two [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	90.00
Oil [Member] | 2015 Hedging Position Three [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	90.25
Oil [Member] | 2015 Hedging Position Four [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	90.40
Oil [Member] | 2015 Hedging Position Five [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	91.05
Oil [Member] | 2015 Hedging Position Six [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	93.28
Oil [Member] | 2015 Hedging Position Seven [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	93.37
Oil [Member] | 2015 Hedging Position Eight [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	94.85
Oil [Member] | 2015 Hedging Position Nine [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	95.00
Oil [Member] | 2016 Hedging Position One [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	1,000
Swap Price	90.00
Natural Gas [Member] | 2014 Hedging Position One [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.000
Natural Gas [Member] | 2014 Hedging Position Two [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.040
Natural Gas [Member] | 2014 Hedging Position Three [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.105
Natural Gas [Member] | 2014 Hedging Position Four [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.190
Natural Gas [Member] | 2014 Hedging Position Five [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.250
Natural Gas [Member] | 2014 Hedging Position Six [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.250
Natural Gas [Member] | 2014 Hedging Position Seven [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.350
Natural Gas [Member] | 2015 Hedging Position One [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.005
Natural Gas [Member] | 2015 Hedging Position Two [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.120
Natural Gas [Member] | 2015 Hedging Position Three [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.150
Natural Gas [Member] | 2015 Hedging Position Four [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.165
Natural Gas [Member] | 2015 Hedging Position Five [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.220
Natural Gas [Member] | 2015 Hedging Position Six [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.255
Natural Gas [Member] | 2016 Hedging Position One [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.110
Natural Gas [Member] | 2016 Hedging Position Two [Member]
Derivatives, Fair Value [Line Items]
Daily Volume	10,000
Swap Price	4.120