Long-Term Debt (Details) (USD $)	3 Months Ended							3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended		3 Months Ended
	Mar. 31, 2012 Bank	Mar. 31, 2011	Dec. 31, 2011	Nov. 30, 2011	May 31, 2011	May 31, 2012 Before Increase [Member]	May 31, 2012 After Increase [Member]	Mar. 31, 2012 Bank Borrowings [Member]	Dec. 31, 2011 Bank Borrowings [Member]	Mar. 31, 2012 Bank Borrowings [Member] After Increase [Member]	Mar. 31, 2012 Senior notes due 2020 [Member]	Mar. 31, 2011 Senior notes due 2020 [Member]	Dec. 31, 2011 Senior notes due 2020 [Member]	Mar. 31, 2012 Senior notes due 2017 [Member]	Mar. 31, 2011 Senior notes due 2017 [Member]	Dec. 31, 2011 Senior notes due 2017 [Member]	Mar. 31, 2012 Senior Notes Due 2022 [Member]	Dec. 31, 2011 Senior Notes Due 2022 [Member]	Mar. 31, 2012 Line of Credit [Member]	Mar. 31, 2011 Line of Credit [Member]
Long-term debt
Long-Term Debt	$ 719,875,000		$ 719,775,000								$ 221,940,000		$ 221,873,000	$ 250,000,000		$ 250,000,000	$ 247,935,000	$ 247,902,000
Long Term Debt (Textual) [Abstract]
Maturities on long-term debt											225,000,000			250,000,000			250,000,000
Maximum availability aggregate facility amount					500,000,000					700,000,000
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%
Commitment fee associated with the unfunded portion of the borrowing base	0.50%
Redemption price plus accrued and unpaid interest											104.438%			103.563%			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%
Qualified offerings of our equity at a redemption price of principal amount of notes											108.875%						107.875%
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	13,465,000	8,388,000									5,200,000	5,100,000		4,600,000	4,500,000		5,000,000		600,000	600,000
Senior notes, issued											225,000,000			250,000,000			250,000,000
Stated Rate of Senior notes											8.875%			7.125%			7.875%
Percentage at which senior notes are issued, of par value											98.389%			100.00%			99.156%
Effective yield to maturity											9.125%						8.00%
Unamortized issuance discount on senior notes due 2022											3,600,000						2,100,000
Maturity date of senior notes											Jan. 15, 2020			Jun. 01, 2017			Mar. 01, 2022
Frequency of senior notes interest payments											semi-annually on January 15 and July 15			semi-annually on June 1 and December 1			semi-annually on March 1 and September 1
Commencement of senior notes payment											November 25, 2009			December 1, 2007			March1,2012
Capitalization of debt issuance cost											4,300,000			2,500,000			4,600,000		3,300,000
Borrowing base	325,000,000					325,000,000	375,000,000
Long Term Debt (Additional Textual) [Abstract]
Capitalized interest on our unproved properties	2,000,000	1,900,000
Addition Line of Credit Facility Maximum Borrowing Capacity					200,000,000
Number of syndicated banks	10
Current Borrowing base	325,000,000					325,000,000	375,000,000
Line of Credit Facility, increase in borrowing base	400,000,000
Amount outstanding under credit facility	0		0
Lead bank's prime rate	3.25%
Maximum level of cash dividends in any fiscal year	15,000,000
Aggregate limitation on purchases of stock	50,000,000
Redetermination of borrowing base amount				Every six months
Commitment fees included in interest expense, net	$ 400,000	$ 400,000