Long-Term debt (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	May 31, 2011	Dec. 31, 2010
Long-term debt
Long-Term Debt	$ 471,746,000		$ 471,746,000			$ 471,624,000
Long Term Debt (Textuals) [Abstract]
Interest expense including commitment fees and amortization of debt issuance costs	8,622,000	8,214,000	17,010,000	16,540,000
Capitalization of debt issuance cost	700,000		700,000			3,600,000
Long Term Debt Additional (Textuals) [Abstract]
Capitalized interest on our unproved properties	1,900,000	1,900,000	3,700,000	3,700,000
Renewed and extended credit facility through May 12, 2016					500,000,000
Number of syndicated banks	10		10
Line of Credit Facility, Maximum Increase in Borrowing Capacity			up to $700.0 million
Amount outstanding under credit facility	0		0			0
Line of credit facility, interest rate description			either (a) the lead bank’s prime plus an applicable margin or (b) Eurodollar rate plus an applicable margin. However with respect to (a), if the lead bank’s prime rate is not higher than each of the federal funds rate plus ½%, and the adjusted London Interbank Offered Rate (“LIBOR”) plus 1%, the greatest of these three rates will then apply
Condition on applicable margins for LIBOR loans	vary depending on the level of outstanding debt with escalating rates of 50 to 150 basis points above the Alternative Base Rate and escalating rates of 150 to 250 basis points for Eurodollar rate loans		vary depending on the level of outstanding debt with escalating rates of 50 to 150 basis points above the Alternative Base Rate and escalating rates of 150 to 250 basis points for Eurodollar rate loans
Commitment fee associated with the unfunded portion of the borrowing base			50 basis point
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	At least every six months		At least every six months
Commitment fees included in interest expense, net	400,000	300,000	700,000	700,000
Applicable margins for LIBOR loans that vary depending on the level of outstanding debt with escalating rates above the Alternative Base Rate	50 to 150 basis points
Applicable margins for LIBOR loans that vary depending on the level of outstanding debt with escalating rates above the Eurodollar rate loans	150 to 250 basis points
Before Increase [Member]
Long Term Debt (Textuals) [Abstract]
Borrowing base	300,000,000		300,000,000
After Increase [Member]
Long Term Debt (Textuals) [Abstract]
Borrowing base	400,000,000		400,000,000
Bank Borrowings [Member]
Long-term debt
Long-Term Debt	0		0			0
Senior notes due 2017 [Member]
Long-term debt
Long-Term Debt	250,000,000		250,000,000			250,000,000
Long Term Debt (Textuals) [Abstract]
Maturities on long-term debt	250,000,000		250,000,000
Interest expense including commitment fees and amortization of debt issuance costs	4,500,000	4,500,000	9,100,000	9,100,000
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			December1,2007
Capitalization of debt issuance cost	4,200,000		4,200,000
Holder right to require company to Repurchase notes at a purchase price in cash			Equal to 101% of the principal amount, plus accrued and unpaid interest to the date of purchase.
Redemption price of senior notes on or after June 1, 2012			Redemption price, plus accrued and unpaid interest, of 103.563% of principal, declining in twelve-month intervals to 100% in 2015 and thereafter
Senior notes due 2020 [Member]
Long-term debt
Long-Term Debt	221,746,000		221,746,000			221,624,000
Long Term Debt (Textuals) [Abstract]
Maturities on long-term debt	225,000,000		225,000,000
Interest expense including commitment fees and amortization of debt issuance costs	5,100,000	5,100,000	10,300,000	10,300,000
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			January 15,2010
Redemption price of senior notes on or after January 15, 2013			redemption price, plus accrued and unpaid interest, of 104.438% of principal, declining in twelve-month intervals to 100% in 2018 and thereafter
Redemption price of senior notes prior to January 15, 2015			Redeem up to 35% of the principal amount of the notes with the net proceeds of qualified offerings of our equity at a redemption price of 108.875% of the principal amount of the notes, plus accrued and unpaid interest.
Capitalization of debt issuance cost	5,000,000		5,000,000
Holder right to require company to Repurchase notes at a purchase price in cash			Equal to 101% of the principal amount, plus accrued and unpaid interest to the date of purchase.
Line of Credit [Member]
Long Term Debt (Textuals) [Abstract]
Interest expense including commitment fees and amortization of debt issuance costs	$ 600,000	$ 400,000	$ 1,200,000	$ 900,000