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Derivative Instruments - Schedule of Outstanding Commodity Derivative Instruments (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Derivative [Line Items]  
Fair Value Asset (Liability) $ (14,133)
2013 Two-Way Collars [Member]
  
Derivative [Line Items]  
Settlement Period 2013
Total Notional Amount of Oil (Barrels) 836,000
Average Floor Price (in dollars per barrel) 92.11
Average Ceiling Price (in dollars per barrel) 103.45
Fair Value Asset (Liability) (2,636)
2013 Three-Way Collars [Member]
  
Derivative [Line Items]  
Settlement Period 2013
Total Notional Amount of Oil (Barrels) 832,330
Average Sub-Floor Price (in dollars per barrel) 67.63
Average Floor Price (in dollars per barrel) 92.01
Average Ceiling Price (in dollars per barrel) 110.97
Fair Value Asset (Liability) (118)
2013 Put Spreads [Member]
  
Derivative [Line Items]  
Settlement Period 2013
Total Notional Amount of Oil (Barrels) 168,670
Average Sub-Floor Price (in dollars per barrel) 70.89
Average Floor Price (in dollars per barrel) 90.89
Fair Value Asset (Liability) 4
2013 Swaps [Member]
  
Derivative [Line Items]  
Settlement Period 2013
Total Notional Amount of Oil (Barrels) 880,500
Average Swap Price (in dollars per barrel) 97.29
Fair Value Asset (Liability) (5,118)
2014 Two-Way Collars [Member]
  
Derivative [Line Items]  
Settlement Period 2014
Total Notional Amount of Oil (Barrels) 1,510,000
Average Floor Price (in dollars per barrel) 90.77
Average Ceiling Price (in dollars per barrel) 102.06
Fair Value Asset (Liability) (159)
2014 Three-Way Collars [Member]
  
Derivative [Line Items]  
Settlement Period 2014
Total Notional Amount of Oil (Barrels) 3,530,530
Average Sub-Floor Price (in dollars per barrel) 70.30
Average Floor Price (in dollars per barrel) 90.65
Average Ceiling Price (in dollars per barrel) 105.64
Fair Value Asset (Liability) 2,497
2014 Put Spreads [Member]
  
Derivative [Line Items]  
Settlement Period 2014
Total Notional Amount of Oil (Barrels) 11,470
Average Sub-Floor Price (in dollars per barrel) 70.00
Average Floor Price (in dollars per barrel) 90.00
Fair Value Asset (Liability) 10
2014 Swaps [Member]
  
Derivative [Line Items]  
Settlement Period 2014
Total Notional Amount of Oil (Barrels) 2,218,500
Average Swap Price (in dollars per barrel) 95.87
Fair Value Asset (Liability) (2,486)
2014 Swaps with sub-floors [Member]
  
Derivative [Line Items]  
Settlement Period 2014
Total Notional Amount of Oil (Barrels) 2,004,000
Average Swap Price (in dollars per barrel) 92.60
Average Sub-Floor Price (in dollars per barrel) 70.00
Fair Value Asset (Liability) (7,202)
2015 Two-Way Collars [Member]
  
Derivative [Line Items]  
Settlement Period 2015
Total Notional Amount of Oil (Barrels) 108,500
Average Floor Price (in dollars per barrel) 90.00
Average Ceiling Price (in dollars per barrel) 99.86
Fair Value Asset (Liability) 284
2015 Three-Way Collars [Member]
  
Derivative [Line Items]  
Settlement Period 2015
Total Notional Amount of Oil (Barrels) 263,500
Average Sub-Floor Price (in dollars per barrel) 70.59
Average Floor Price (in dollars per barrel) 90.59
Average Ceiling Price (in dollars per barrel) 105.25
Fair Value Asset (Liability) 723
2015 Swaps [Member]
  
Derivative [Line Items]  
Settlement Period 2015
Total Notional Amount of Oil (Barrels) 108,500
Average Swap Price (in dollars per barrel) 93.07
Fair Value Asset (Liability) 148
2015 Swaps with sub-floors [Member]
  
Derivative [Line Items]  
Settlement Period 2015
Total Notional Amount of Oil (Barrels) 186,000
Average Swap Price (in dollars per barrel) 92.60
Average Sub-Floor Price (in dollars per barrel) 70.00
Fair Value Asset (Liability) $ (80)