Long-Term Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 Bank		Dec. 31, 2010		Dec. 31, 2009	Nov. 30, 2011	May 31, 2011
Long-term debt
Long-Term Debt	$ 719,775,000	[1]	$ 471,624,000	[1]
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	35,566,000		33,437,000		30,663,000
Long Term Debt Additional (Textual) [Abstract]
Capitalized interest on our unproved properties	7,700,000		7,400,000		6,100,000
Renewed and extended credit facility through May 12, 2016							500,000,000
Addition Line of Credit Facility Maximum Borrowing Capacity							200,000,000
Number of syndicated banks	10
Current Borrowing base						300,000,000
Increased borrowing base						400,000,000
Decreased borrowing base						325,000,000
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	1,500,000		1,400,000		700,000
Before Increase [Member]
Long Term Debt (Textual) [Abstract]
Borrowing base	300,000,000
After Increase [Member]
Long Term Debt (Textual) [Abstract]
Borrowing base	400,000,000
Bank Borrowings [Member]
Long-term debt
Long-Term Debt	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
Long Term Debt Additional (Textual) [Abstract]
Renewed and extended credit facility through May 12, 2016	700,000,000
Senior notes due 2017 [Member]
Long-term debt
Long-Term Debt	250,000,000		250,000,000
Long Term Debt (Textual) [Abstract]
Maturities on long-term debt	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%
Holder right to require company to repurchase 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	18,200,000		18,400,000		18,100,000
Senior notes, issued	250,000,000
Senior notes stated percentage	7.125%
Percentage at which senior notes are issued net of discount	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
Senior notes due 2020 [Member]
Long-term debt
Long-Term Debt	221,873,000	[1]	221,624,000	[1]
Long Term Debt (Textual) [Abstract]
Maturities on long-term debt	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%
Holder right to require company to repurchase 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	20,600,000		20,500,000		2,000,000
Senior notes, issued	225,000,000
Senior notes stated percentage	8.875%
Percentage at which senior notes are issued net of discount	98.389%
Effective yield to maturity	9.125%
Unamortized issuance discount on senior notes due 2020	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
Senior Notes Due 2022 [Member]
Long-term debt
Long-Term Debt	247,902,000	[1]	0	[1]
Long Term Debt (Textual) [Abstract]
Maturities on long-term debt	250,000,000
Redemption price plus accrued and unpaid interest	103.938%
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	107.875%
Holder right to require company to repurchase 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	1,700,000
Senior notes, issued	250,000,000
Senior notes stated percentage	7.875%
Percentage at which senior notes are issued net of discount	99.156%
Effective yield to maturity	8.00%
Unamortized issuance discount on senior notes due 2020	2,100,000
Maturity date of senior notes	Mar. 01, 2022
Frequency of senior notes interest payments	semi-annually on March 1 and September 1
Commencement of senior notes payment	March1,2012
Capitalization of debt issuance cost	4,800,000
Senior Notes Due 2011 [Member]
Long Term Debt (Textual) [Abstract]
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility					11,400,000
Senior notes, issued	150,000,000
Senior notes stated percentage	7.625%
Capitalization of debt issuance cost					1,100,000
Redemption amount of senior notes	4,000,000
Premium on Redemption Of Senior Notes	2,900,000
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	$ 2,400,000		$ 1,900,000		$ 5,200,000

[1] Amounts are shown net of debt discount