Terra Nova Working Interest Redetermination - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010		Sep. 30, 2011	Sep. 30, 2010		Dec. 31, 2010
Commitments and Contingencies Disclosure [Line Items]
Redetermination of Terra Nova working interest	$ 0	$ (4,491)	[1]	$ 5,351	$ (15,353)	[1]	$ 102,100
Joint Operating Agreement
Commitments and Contingencies Disclosure [Line Items]
Joint agreement ownership interest	12.00%			12.00%
End of Period
Commitments and Contingencies Disclosure [Line Items]
Joint agreement ownership interest	10.475%			10.475%

[1] Reclassified to conform to current presentation