Property Acquisition and Dispositions - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended			1 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012 acre	Feb. 29, 2012 acre
Property Acquisition And Dispositions [Line Items]
Acquired producing properties in cash		$ 458,446,000	$ 563,485,000	$ 1,484,016,000
Acquisitions and disposals proceeds	90,000,000			126,400,000	126,400,000
Recognized pre-tax gain				68,000,000	68,000,000
Asset retirement obligations for disposed properties				8,300,000	8,300,000
Interest Sold	49.90%
Net Acres Sold	44,000
Amount of capital to be funded by partner	270,000,000
Percent of capital funded by partner	50.00%
NORTH DAKOTA
Property Acquisition And Dispositions [Line Items]
Acquired producing properties in cash					663,300,000	276,000,000
Acres acquired				119,000	119,000	23,100
NORTH DAKOTA | Producing Properties [Member]
Property Acquisition And Dispositions [Line Items]
Daily production of acquired producing properties, barrels of oil per day				6,500	6,500	1,000
NORTH DAKOTA | Producing properties [Member]
Property Acquisition And Dispositions [Line Items]
Acquired producing properties in cash					477,100,000	51,700,000
Wyoming [Member]
Property Acquisition And Dispositions [Line Items]
Acquisitions and disposals proceeds						84,400,000
Recognized pre-tax gain						50,100,000
Asset retirement obligations for disposed properties						11,100,000
Oklahoma [Member]
Property Acquisition And Dispositions [Line Items]
Acquisitions and disposals proceeds				15,900,000	15,900,000
Recognized pre-tax gain				15,900,000	15,900,000
Asset retirement obligations for disposed properties				$ 600,000	$ 600,000