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				1 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Jul. 31, 2012	Aug. 31, 2011 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Sep. 30, 2012 Revolving credit facility	Sep. 30, 2012 Revolving credit facility Minimum	Sep. 30, 2012 Revolving credit facility Maximum	Sep. 30, 2012 Revolving credit facility After June 30, 2013	Sep. 30, 2012 Revolving credit facility Future Period Two [Member]	Sep. 30, 2012 Revolving credit facility Group Twelve	Sep. 30, 2012 Revolving credit facility Group Thirteen	Aug. 31, 2011 Revolving credit facility Letter of Credit	Sep. 30, 2012 10.375% Senior Unsecured Notes due 2016	Dec. 31, 2011 10.375% Senior Unsecured Notes due 2016	Sep. 30, 2012 10.375% Senior Unsecured Notes due 2016 Semi Annual Payment, First Payment	Sep. 30, 2012 10.375% Senior Unsecured Notes due 2016 Semi Annual Payment, Second Payment	Sep. 30, 2012 10.375% Senior Unsecured Notes due 2016 Group One	Sep. 30, 2012 10.375% Senior Unsecured Notes due 2016 Group Two	Apr. 30, 2011 4.50% Convertible Notes due 2012	Sep. 30, 2012 4.50% Convertible Notes due 2012	Dec. 31, 2011 4.50% Convertible Notes due 2012	Sep. 30, 2012 Previous Revolving Credit Facility	Sep. 30, 2012 7.25% Senior Unsecured Notes due 2019	Sep. 30, 2012 7.25% Senior Unsecured Notes due 2019 Group One	Sep. 30, 2012 7.25% Senior Unsecured Notes due 2019 Group Two
Debt Disclosure [Line Items]
Maximum borrowing capacity					$ 300,000,000									$ 20,000,000
Optional additional borrowing					300,000,000
Borrowing Base Commitment Description					The Revolver has an accordion feature that allows us to increase the commitment up to the lower of the borrowing base or $600.0 million upon receiving additional commitments from one or more lenders. The Revolver has a borrowing base that is redetermined semi-annually. In April 2012, the bank syndicate supporting the Revolver completed its redetermination and established a borrowing base of $300.0 million. Accordingly, the minimum revolving commitment remained unchanged.
Established borrowing base				300,000,000
Letter of credit amount outstanding																								1,600,000
Available borrowing capacity																								221,400,000
Payments of Debt Issuance Costs	1,900,000	1,779,000	8,850,000
Debt instrument, debt issuance costs, amortization period	5 years
Write off of Deferred Debt Issuance Cost	3,200,000
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.
Credit facility interest rate option one, applicable margin rate over Adjusted LIBOR						1.75%		1.50%	2.50%
Credit facility interest rate option two, base rate over federal funds effective rate							0.50%
Debt Instrument, Description of Variable Rate Basis		one-month Adjusted LIBOR
Credit facility interest rate option two, base rate over one-month Adjusted LIBOR							1.00%
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																						8.50%
Principal amount															300,000,000	300,000,000						4,915,000	4,915,000
Current ratio								1	1
Debt to EBITDAX ratio							4.5000			4.25000	4
Original sale amount, percentage of par															97.00%
Effective yield to maturity															11.00%
Annual interest rate	4.00%	4.00%													10.375%							4.50%			7.25%
Interest payment date																	June 15	December 15
Redemption period start																			June 2013	June 2015
Redemption option, percent of principal																			105.188%	100.00%						103.625%	100.00%
Conversion rate, numerator																						17.3160000
Conversion rate, denominator																						1,000
Initial conversion price																						$ 57.75
Principal amount of Convertible notes tendered																					225,100,000
Cash paid to repurchase principal																					233,000,000
Tender premium per principal amount																					35
Tender offer, principal amount																					$ 1,000
Percentage of outstanding debt extinguished																					98.00%