Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Y	Nov. 30, 2010	Dec. 31, 2011
Long-term Purchase Commitment [Line Items]
Total future drilling commitments at balance sheet date	$ 95,000,000
Drilling commitments 2013	80,000,000
Drilling commitments 2014	15,000,000
Length of extension offered on hydraulic fracturing services contract, years	1
Length of hydraulic fracturing services contract, years	3
Future remaining commitments in 2013	17,000,000
Loss related to contingency damages		145
Legal proceedings recorded as a liability under other noncurrent liabilities	2,400,000		2,600,000
Pipeline Transportation Commitments [Member]
Long-term Purchase Commitment [Line Items]
Transportation commitment, in years	5 years
Barrels of crude oil per day	15,000
Rail commitments, end date	2017-11
Transportation commitment, total	55,000,000
Transportation commitment, due in 2013	13,000,000
Transportation commitment, due in 2013	13,000,000
Transportation commitment, due in 2014	10,000,000
Transportation commitment, due in 2015	5,000,000
Transportation commitment, due in 2017	5,000,000
Non-operational [Member]
Long-term Purchase Commitment [Line Items]
Transportation commitment, total	1,000,000,000
Rail Transportation Commitments [Member]
Long-term Purchase Commitment [Line Items]
Rail commitments, end date	2015-12
Transportation commitment, total	52,000,000
Transportation commitment, due in 2013	35,000,000
Transportation commitment, due in 2014	10,000,000
Transportation commitment, due in 2015	$ 7,000,000
Rail Transportation Commitments [Member] | Minimum [Member]
Long-term Purchase Commitment [Line Items]
Number of barrels of crude oil, per day	2,500
Rail Transportation Commitments [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Number of barrels of crude oil, per day	10,000