Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended		9 Months Ended			9 Months Ended				9 Months Ended					9 Months Ended				1 Months Ended	3 Months Ended	9 Months Ended			1 Months Ended
	Sep. 30, 2011	Sep. 30, 2011	Aug. 31, 2011 Revolving credit facility	Sep. 30, 2011 Revolving credit facility	Sep. 30, 2011 Revolving credit facility	Aug. 31, 2011 Revolving credit facility Letter of Credit	Sep. 30, 2011 Revolving credit facility Commitment Fee Percentage on the Undrawn Portion of the Facility at Current Utilization Levels	Sep. 30, 2011 Revolving credit facility Commitment Fee Percentage Based upon Utilization Levels in Excess of 50% of the Borrowing Base	Sep. 30, 2011 Revolving credit facility After September 30, 2013	Sep. 30, 2011 Revolving credit facility Minimum	Sep. 30, 2011 Revolving credit facility Maximum	Sep. 30, 2011 Previous Revolving Credit Facility	Sep. 30, 2011 10.375% Senior Unsecured Notes due 2016	Sep. 30, 2011 10.375% Senior Unsecured Notes due 2016 Semi Annual Payment, First Payment	Sep. 30, 2011 10.375% Senior Unsecured Notes due 2016 Semi Annual Payment, Second Payment	Sep. 30, 2011 10.375% Senior Unsecured Notes due 2016 Group one	Sep. 30, 2011 10.375% Senior Unsecured Notes due 2016 Group Two	Sep. 30, 2011 7.25% Senior Unsecured Notes due 2019	Sep. 30, 2011 7.25% Senior Unsecured Notes due 2019 Semi Annual Payment, First Payment	Sep. 30, 2011 7.25% Senior Unsecured Notes due 2019 Semi Annual Payment, Second Payment	Sep. 30, 2011 7.25% Senior Unsecured Notes due 2019 Group one	Sep. 30, 2011 7.25% Senior Unsecured Notes due 2019 Group Two	Apr. 30, 2011 4.5% Convertible Notes due 2012	Jun. 30, 2011 4.5% Convertible Notes due 2012	Sep. 30, 2011 4.5% Convertible Notes due 2012	Sep. 30, 2011 4.5% Convertible Notes due 2012 Semi Annual Payment, First Payment	Sep. 30, 2011 4.5% Convertible Notes due 2012 Semi Annual Payment, Second Payment	Aug. 31, 2011 Maximum
Debt Disclosure [Line Items]
Maximum borrowing capacity			$ 300,000,000			$ 20,000,000
Credit facility expiration			August 2016
Letter of credit initial borrowing base			380,000,000
Optional additional borrowing			600,000,000
Borrowing Base Commitment Description			The borrowing base is redetermined semi-annually. There is an accordion feature that allows us to increase the commitment up to the lower of the borrowing base or $600 million upon receiving additional commitments from one or more lenders.
Credit facility interest rate description					Borrowings under the Revolver bear interest, at our option, at either (i) a rate derived from the London Interbank Offered Rate, as adjusted for statutory reserve requirements for Eurocurrency liabilities (the "Adjusted LIBOR"), plus an applicable margin ranging from 1.500% to 2.500% or (ii) the greater of (a) the prime rate, (b) the federal funds effective rate plus 0.5% or (c) the one-month Adjusted LIBOR plus 1.0%, and, in each case, plus an applicable margin (ranging from 0.500% to 1.500%). The applicable margin is determined based on the ratio of our outstanding borrowings to the available Revolver capacity. Commitment fees are charged at 0.375% increasing to 0.500% on the undrawn portion of the Revolver as determined by our ratio of outstanding borrowings to the available Revolver capacity.
Credit facility interest rate option two, base rate over federal funds effective rate					0.50%
Credit facility interest rate option two, base rate over one-month Adjusted LIBOR					1.00%
Credit facility interest rate option one, applicable margin rate over Adjusted LIBOR										1.50%	2.50%
Credit facility interest rate option two, applicable margin rate										0.50%	1.50%
Commitment fees for undrawn credit facility							0.375%	0.50%
Effective interest rate				1.80%																					8.50%
Current ratio										1
Debt to EBITDAX ratio				4.5	4.5				4
Letter of credit amount outstanding												1,400,000
Available borrowing capacity												284,000,000
Original sale amount, percentage of par													97.00%
Effective yield to maturity													11.00%
Annual interest rate													10.375%					7.25%							4.50%
Interest payment date														June 15	December 15				April 15	October 15						May 15	November 15
Redemption period start																June 2013	June 2015				April 2015	June 2017
Redemption option, percent of principal																105.188%	100.00%				103.625%	100.00%
Debt maturity																									November 2012
Conversion rate, numerator																									17.3160
Conversion rate, denominator																									1,000
Initial conversion price																									$ 57.75
Principal amount of Convertible Notes tendered																							225,085,000	225,085,000
Cash paid to repurchase principal																							232,963,000	232,963,000
Tender premium per principal amount																							35
Tender offer, principal amount																							1,000
Percentage of outstanding debt extinguished																							98.00%
Loss on extinguishment of debt																								(25,900,000)
Loss on extinguishment of debt charged to earnings	(1,165,000)	(25,403,000)																						(24,238,000)
Loss on extinguishment of debt charged directly to shareholders' equity																								(1,700,000)
Outstanding warrants to acquire shares of common stock	3,982,680	3,982,680
Warrants exercise price		74.25
Cash proceeds from sale of Note Hedges																												$ 100,000