Property Acquisition and Dispositions - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended				1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 North Dakota [Member] acre	Feb. 29, 2012 North Dakota [Member] acre	Dec. 31, 2012 North Dakota [Member] Producing Properties [Member] Boe	Feb. 29, 2012 North Dakota [Member] Producing Properties [Member] Boe	Dec. 31, 2012 North Dakota [Member] Producing properties [Member]	Feb. 29, 2012 North Dakota [Member] Producing properties [Member]	Feb. 29, 2012 Wyoming [Member]	Dec. 31, 2012 Oklahoma [Member]	Dec. 31, 2011 Total of Michigan, North Dakota and Montana [Member]
Property Acquisition And Dispositions [Line Items]
Acquired producing and undeveloped properties in cash	$ 563,485,000	$ 1,484,016,000	$ 248,562,000	$ 663,300,000	$ 276,000,000			$ 477,100,000	$ 51,700,000
Acres acquired				119,000	23,100
Daily production of acquired producing properties, barrels of oil per day						6,500	1,000
Acquisitions and disposals proceeds		126,400,000								84,400,000	15,900,000	30,200,000
Recognized pre-tax gain		68,000,000								50,100,000	15,900,000	21,400,000
Asset retirement obligations for disposed properties		$ 8,300,000								$ 11,100,000	$ 600,000