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Long-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified			12 Months Ended				12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Senior secured revolving line of credit [Member]	Dec. 31, 2012 Senior unsecured notes [Member]	Dec. 31, 2012 Amended Credit Facility [Member]	Oct. 02, 2012 Amended Credit Facility [Member]	Apr. 03, 2012 Amended Credit Facility [Member]	Dec. 31, 2012 Amended Credit Facility [Member] Minimum [Member] EBITDAX [Member]	Dec. 31, 2012 Amended Credit Facility [Member] Minimum [Member] Current Ratio [Member]	Dec. 31, 2011 2021 and 2019 Notes [Member]	Jul. 02, 2012 2023 Notes [Member]	Dec. 31, 2011 2019 Notes [Member]	Dec. 31, 2011 2021 Notes [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						783.4	392.4
Debt instrument, maturity date				Oct. 06, 2016						Jan. 15, 2023	Feb. 01, 2019	Nov. 01, 2021
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 unused borrowing base capacity				497.8
Letter of credit borrowings		1,000			500	350
Letter of credit maximum borrowings					750
Minimum debt to EBITDAX ratio							2.5
Minimum current ratio								1.0
Commitment fee of borrowing base capacity not utilized				0.375%
Outstanding letters of credit				2.2
Deferred financing costs	$ 23.5

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