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Long-Term Debt (Narrative) (Detail) (USD $) In Millions, unless otherwise specified			9 Months Ended		1 Months Ended	12 Months Ended						9 Months Ended
	Sep. 30, 2012	Oct. 06, 2011 Line of Credit [Member]	Sep. 30, 2012 Unsecured Senior Notes [Member]	Sep. 30, 2012 Amended Credit Facility [Member]	Jul. 02, 2012 2023 Notes [Member]	Dec. 31, 2011 2019 Notes [Member]	Dec. 31, 2011 2021 Notes [Member]	Dec. 31, 2011 2021 and 2019 Notes [Member]	Apr. 03, 2012 Maximum [Member] Amended Credit Facility [Member]	Sep. 30, 2012 Maximum [Member] Amended Credit Facility [Member] EBITDAX [Member]	Oct. 06, 2011 Minimum [Member] Amended Credit Facility [Member]	Sep. 30, 2012 Minimum [Member] Amended Credit Facility [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		392.4			783.4
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
Letters of credit borrowings		1,000							500		350
Percentage of the reserve value of assets as determined by reserve reports				80.00%
Commitment fee of borrowing base capacity not utilized				0.375%
Maximum debt to EBITDAX ratio										4
Minimum current ratio												1.0
Letter of credit unused borrowing base capacity				500
Deferred financing costs	$ 24.3

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Interest rate stated in the contractual debt agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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