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Derivative Instruments (Schedule of Outstanding Commodity Derivative Instruments) (Detail) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended
Sep. 30, 2012
bbl
Derivative Instruments [Line Items]  
Fair Value Asset (Liability) $ 25,734
2012 Two-Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 819,000
Average Floor Price 88.61
Average Ceiling Price 105.59
Fair Value Asset (Liability) 975
Settlement Period 2012
2012 Three-Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 910,000
Average Sub-Floor Price 66.25
Average Floor Price 90.25
Average Ceiling Price 110.04
Fair Value Asset (Liability) 841
Settlement Period 2012
Two Thousand Twelve Swap Contracts [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 91,000
Average Swap Price 94.61
Fair Value Asset (Liability) 130
Settlement Period 2012
2013 Two-Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 1,782,000
Average Floor Price 86.50
Average Ceiling Price 98.93
Fair Value Asset (Liability) (1,164)
Settlement Period 2013
2013 Three-Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 2,023,420
Average Sub-Floor Price 65.30
Average Floor Price 92.51
Average Ceiling Price 112.63
Fair Value Asset (Liability) 8,561
Settlement Period 2013
2013 Put Spreads [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 1,717,080
Average Sub-Floor Price 70.71
Average Floor Price 91.24
Fair Value Asset (Liability) 9,931
Settlement Period 2013
Two Thousand Thirteen Swap Contracts [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 699,000
Average Swap Price 96.41
Fair Value Asset (Liability) 1,850
Settlement Period 2013
2014 Two-Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 139,500
Average Floor Price 86.11
Average Ceiling Price 97.69
Fair Value Asset (Liability) (84)
Settlement Period 2014
2014 Three-Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 1,495,030
Average Sub-Floor Price 70.60
Average Floor Price 91.43
Average Ceiling Price 110.30
Fair Value Asset (Liability) 3,229
Settlement Period 2014
2014 Put Spreads [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 150,970
Average Sub-Floor Price 71.03
Average Floor Price 91.03
Fair Value Asset (Liability) 1,019
Settlement Period 2014
Two Thousand Fourteen Swap Contracts [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 62,000
Average Swap Price 96.49
Fair Value Asset (Liability) 215
Settlement Period 2014
2015 Three Way Collars [Member]
  
Derivative Instruments [Line Items]  
Total Notional Amount of Oil (Barrels) 124,000
Average Sub-Floor Price 71.25
Average Floor Price 91.25
Average Ceiling Price 109.96
Fair Value Asset (Liability) $ 231
Settlement Period 2015