Long-Term Debt (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011 Year Bank	Sep. 30, 2010	Sep. 30, 2011 Year Installment Bank	Sep. 30, 2010	May 31, 2011	Dec. 31, 2010
Long-term debt
Long-Term Debt	$ 471,809,000		$ 471,809,000			$ 471,624,000
Long Term Debt (Textual) [Abstract]
Maximum availability aggregate facility amount					500,000,000
Commitment fee associated with the unfunded portion of the borrowing base			0.50%
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	8,439,000	8,264,000	25,449,000	24,804,000
Capitalization of debt issuance cost	700,000		700,000
Long Term Debt Additional (Textual) [Abstract]
Capitalized interest on our unproved properties	2,000,000	1,900,000	5,700,000	5,500,000
Renewed and extended credit facility through May 12, 2016					500,000,000
Number of syndicated banks	10		10
Amount outstanding under credit facility	0		0			0
Lead bank's prime rate	3.25%		3.25%
Maximum level of cash dividends in any fiscal year	15,000,000		15,000,000
Aggregate limitation on purchases of stock	50,000,000		50,000,000
Redetermination of borrowing base amount	Every six months		Every six months
Commitment fees included in interest expense, net	400,000	400,000	1,100,000	1,000,000
Bank Borrowings [Member]
Long-term debt
Long-Term Debt	0		0			0
Long Term Debt (Textual) [Abstract]
Percentage of applicable margin with federal fund rate			0.50%
Percentage of applicable margin with LIBOR			1.00%
Maximum escalating rates on applicable margins depending upon outstanding debt			1.50%
Minimum escalating rates on applicable margins depending upon outstanding debt			0.50%
Minimum escalating rates for Eurodollar rate loans			1.50%
Maximum escalating rates for Eurodollar rate loans			2.50%
Bank Borrowings [Member] | After Increase [Member]
Long Term Debt (Textual) [Abstract]
Maximum availability aggregate facility amount	700,000,000		700,000,000
Long Term Debt Additional (Textual) [Abstract]
Renewed and extended credit facility through May 12, 2016	700,000,000		700,000,000
Senior notes due 2017 [Member]
Long-term debt
Long-Term Debt	250,000,000		250,000,000			250,000,000
Long Term Debt (Textual) [Abstract]
Maturities on long-term debt	250,000,000		250,000,000
Redemption price plus accrued and unpaid interest			103.563%
Redemption price, plus accrued and unpaid interest declining in twelve-month intervals			100.00%
Repurchase all or any part of notes at a purchase price in cash			101.00%
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	4,500,000	4,500,000	13,600,000	13,600,000
Senior notes, issued	250,000,000		250,000,000
Senior notes stated percentage	7.125%		7.125%
Percentage at which senior notes are issued net of discount	100.00%		100.00%
Maturity date of senior notes			Jun. 01, 2017
Frequency of senior notes interest payments			semi-annually on June 1 and December 1
Commencement of senior notes payment			December 1, 2007
Capitalization of debt issuance cost	4,200,000		4,200,000
Senior notes due 2020 [Member]
Long-term debt
Long-Term Debt	221,809,000		221,809,000			221,624,000
Long Term Debt (Textual) [Abstract]
Maturities on long-term debt	225,000,000		225,000,000
Redemption price plus accrued and unpaid interest			104.438%
Redemption price, plus accrued and unpaid interest declining in twelve-month intervals			100.00%
Percentage of maximum redemption of principal amount of notes			35.00%
Qualified offerings of our equity at a redemption price of principal amount of notes			108.875%
Repurchase all or any part of notes at a purchase price in cash			101.00%
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	5,100,000	5,100,000	15,400,000	15,400,000
Senior notes, issued	225,000,000		225,000,000
Senior notes stated percentage	8.875%		8.875%
Percentage at which senior notes are issued net of discount	98.389%		98.389%
Effective yield to maturity			9.125%
Unamortized issuance discount on senior notes due 2020	3,600,000		3,600,000
Maturity date of senior notes			Jan. 15, 2020
Frequency of senior notes interest payments			semi-annually on January 15 and July 15
Commencement of senior notes payment			November 25, 2009
Capitalization of debt issuance cost	5,000,000		5,000,000
Line of Credit [Member]
Long Term Debt (Textual) [Abstract]
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	600,000	500,000	1,800,000	1,300,000
Before Increase [Member]
Long Term Debt (Textual) [Abstract]
Borrowing base	300,000,000		300,000,000
After Increase [Member]
Long Term Debt (Textual) [Abstract]
Borrowing base	$ 400,000,000		$ 400,000,000