Commitments and Contingencies - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended
	Nov. 30, 2010	Sep. 30, 2013	Dec. 31, 2012
Long-term Purchase Commitment [Line Items]
Total future drilling commitments at balance sheet date		$ 53,000,000
Drilling commitments remainder of 2013		31,000,000
Drilling commitments 2014		22,000,000
Future Drilling Commitments End Date		2014-08
Loss related to contingency damages	165,000,000
Legal proceedings recorded as a liability under other noncurrent liabilities		(1,700,000)	(2,400,000)
Pipeline Transportation Commitments [Member]
Long-term Purchase Commitment [Line Items]
Barrels of crude oil per day		15,000
Transportation commitment, in years		5 years
Rail commitments, end date		2017-11
Crude oil transportation commitment, total		46,000,000
Crude oil transportation commitment, due in 2013		3,000,000
Crude oil transportation commitment, due in 2014		14,000,000
Crude oil transportation commitment, due in 2015		14,000,000
Crude oil transportation commitment, due in 2016		10,000,000
Crude oil transportation commitment, due in 2017		5,000,000
Pipeline Access Capacity Commitment [Member]
Long-term Purchase Commitment [Line Items]
Transportation commitment, in years		10 years
Natural gas transportation commitment		25,000,000
Non-operational [Member]
Long-term Purchase Commitment [Line Items]
Transportation commitment, in years		5 years
Crude oil non-op transportation commitment, total		1,000,000,000
Crude oil transportation commitment with affiliate		96,000,000
Rail Transportation Commitments [Member]
Long-term Purchase Commitment [Line Items]
Rail commitments, end date		2014-12
Crude oil transportation commitment, total		19,000,000
Crude oil transportation commitment, due in 2013		9,000,000
Crude oil transportation commitment, due in 2014		$ 10,000,000
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