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Long-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended			3 Months Ended			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Senior secured revolving line of credit [Member]	Dec. 31, 2012 2023 Notes [Member]	Dec. 31, 2011 2019 Notes [Member]	Dec. 31, 2011 2021 Notes [Member]	Mar. 31, 2013 Unsecured Debt [Member]	Mar. 31, 2013 Amended Credit Facility [Member]	Oct. 02, 2012 Amended Credit Facility [Member]	Mar. 31, 2013 Amended Credit Facility [Member] Minimum [Member] EBITDAX [Member]	Mar. 31, 2013 Amended Credit Facility [Member] Minimum [Member] Current Ratio [Member]
Debt Disclosure [Line Items]
Senior unsecured debt principal amount				$ 400.0	$ 400.0	$ 400.0
Senior unsecured notes stated interest rate				6.875%	7.25%	6.50%
Senior unsecured notes payment frequency							Semi-annually
Net proceeds from issuance of notes							1,175.8
Debt instrument, maturity date				Jan. 15, 2023	Feb. 01, 2019	Nov. 01, 2021		Oct. 06, 2016
Redemption percentage, maximum							35.00%
Percentage of minimum outstanding principal amount							65.00%
Event of default by payment default							10.0
Event of default by final judgments							10.0
Percentage of the reserve value of assets as determined by reserve reports								80.00%
Letter of credit borrowings			1,000						500
Letter of credit maximum borrowings									750
Commitment fee of borrowing base capacity not utilized								0.375%
Minimum debt to EBITDAX ratio										2.5
Minimum current ratio											1.0
Outstanding letters of credit								2.2
Line of Credit Facility, Remaining Borrowing Capacity								497.8
Deferred financing costs	22.6
Amortization of deferred financing costs	$ 0.8	$ 0.6

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