Property Transaction with Related Party - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 MBoe	Dec. 31, 2011	Mar. 27, 2012 Wheatland Oil Inc. [Member]	Dec. 31, 2012 Wheatland [Member]	Dec. 31, 2012 Oil And Natural Gas [Member]
Related Party Transaction [Line Items]
CEO ownership in related party			75.00%
Vice Chairman ownership in related party			25.00%
Issuance of common stock to acquire property, in shares	3.9
Common stock par value per share	$ 0.01	$ 0.01
Consideration cost	$ 279
Net book value				82	177
Joint interest obligations					38
Asset retirement obligations	0.6
Deferred income tax liabilities	57
Purchase price adjustments arising after the closing date as allowed				0.5
Crude oil and natural gas production	484
Crude oil and natural gas revenues	$ 38