Long-Term Debt (Details) (USD $)	12 Months Ended									12 Months Ended				12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2012 Before Increase [Member]	Sep. 30, 2012 Before Increase [Member]	Dec. 31, 2012 After Increase [Member]	Oct. 31, 2012 After Increase [Member]	Dec. 31, 2012 Bank Borrowings [Member]	Dec. 31, 2011 Bank Borrowings [Member]	Oct. 31, 2012 Bank Borrowings [Member] Before Increase [Member]	Oct. 31, 2012 Bank Borrowings [Member] After Increase [Member] Banks	Dec. 31, 2012 Line of Credit [Member]	Dec. 31, 2011 Line of Credit [Member]	Dec. 31, 2010 Line of Credit [Member]	Dec. 31, 2012 Senior notes due 2017 [Member]	Dec. 31, 2011 Senior notes due 2017 [Member]	Dec. 31, 2010 Senior notes due 2017 [Member]	Dec. 31, 2012 Senior notes due 2020 [Member]		Dec. 31, 2011 Senior notes due 2020 [Member]		Dec. 31, 2010 Senior notes due 2020 [Member]	Dec. 31, 2012 Senior Notes due 2022 [Member]		Dec. 31, 2011 Senior Notes due 2022 [Member]		Oct. 03, 2012 Senior Notes due 2022 [Member]	Dec. 31, 2012 Additional Senior Notes Due 2022 [Member]	Oct. 03, 2012 Additional Senior Notes Due 2022 [Member]
Bank Borrowings
Maturities on long-term debt										$ 39,400,000							$ 250,000,000			$ 225,000,000					$ 400,000,000
Capitalized interest on our unproved properties	7,900,000		7,700,000		7,400,000
Maximum availability aggregate facility amount												500,000,000
Number of syndicated banks													11
Current Borrowing base							375,000,000	450,000,000	450,000,000
Long-Term Debt	916,934,000	[1]	719,775,000	[1]						39,400,000	0						250,000,000	250,000,000		222,147,000	[1]	221,873,000	[1]		405,387,000	[1]	247,902,000	[1]
Current Committed borrowing base amount						450,000,000	300,000,000
Percentage of applicable margin with federal fund rate										0.50%
Percentage of applicable margin with LIBOR										1.00%
Minimum debt instrument escalating basis pread on base rate										50
Maximum escalating rates on applicable margins depending upon outstanding debt										150
Minimum escalating rates for Eurodollar rate loans										150
Maximum escalating rates for Eurodollar rate loans										250
Lead bank's prime rate										3.25%
Amount of current borrowing capacity under the credit facility with a commitment fee of 37.5 basis points										225,000,000
Commitment Fee Basis Points On First $225 Million of Borrowing Base Under The Credit Facility										37.5
Commitment fee basis points on unused line of credit facility										50
Maximum level of cash dividends in any fiscal year														15,000,000
Aggregate limitation on purchases of stock														50,000,000
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	57,303,000		35,566,000		33,437,000									3,700,000	2,400,000	1,900,000	18,200,000	18,200,000	18,400,000	20,600,000		20,600,000		20,500,000	22,400,000		1,700,000
Commitment fees included in interest expense, net														1,400,000	1,500,000	1,400,000
Reduction of credit facility unamortized issuance costs	700,000
Write off of deferred debt issuance cost	700,000
Senior Notes
Senior notes, issued																	250,000,000			225,000,000					400,000,000		250,000,000				150,000,000
Stated rate of senior notes																	7.125%	7.125%		8.875%		8.875%			7.875%		7.875%
Percentage at which senior notes are issued, of par value																	100.00%			98.389%							99.156%				105.00%
Effective interest rate on senior notes including discount																				9.125%							8.00%				6.993%
Original unamortized issuance discount on senior notes																				3,600,000					2,100,000		2,100,000
unamortized premium																														7,500,000
Net cash proceeeds from debt issuance																													156,800,000
Redemption price plus accrued and unpaid interest																	103.563%			104.438%					103.938%
Redemption price, plus accrued and unpaid interest declining in twelve-month intervals																	100.00%			100.00%					100.00%
Percentage of maximum redemption of principal amount of notes																				35.00%					35.00%
Redemption of debts with net proceeds of qualifying offerings at price above principle amount of debts																				108.875%					107.875%
Initial debt issuance costs capitalized																	4,200,000			5,000,000					7,400,000
Holder right to require company to repurchase notes at purchase price in cash																	101.00%			101.00%					101.00%
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	$ 57,303,000		$ 35,566,000		$ 33,437,000									$ 3,700,000	$ 2,400,000	$ 1,900,000	$ 18,200,000	$ 18,200,000	$ 18,400,000	$ 20,600,000		$ 20,600,000		$ 20,500,000	$ 22,400,000		$ 1,700,000

[1] Amounts are shown net of any debt discount