Terra Nova Working Interest Redetermination - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended	9 Months Ended			12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011		Jun. 30, 2011	Sep. 30, 2012	Sep. 30, 2011		Dec. 31, 2010
Commitments and Contingencies Disclosure [Line Items]
Joint agreement ownership interest	10.475%				10.475%			12.00%
Redetermination of Terra Nova working interest	$ 0	$ 0	[1]	$ (5,400)	$ 0	$ (5,351)	[1]	$ 102,100

[1] Reclassified to conform to current presentation.