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Basis of Presentation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Impact fee			$ 24,700,000
Period related to annual impact fee per well drilled			15 years
Abandonment and impairment expense	40,118,000	16,627,000	104,048,000	52,064,000
Southwest Oklahoma And North Texas [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Abandonment and impairment expense			19,600,000
PA unproved impairment [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Abandonment and impairment expense			$ 23,100,000

The adjustment to expense previously capitalized costs of drilling exploratory wells when proved reserves are not found or when the entity obtains information that raises substantial doubt about the economic or operational viability of the project.

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