Long-Term Debt (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Oct. 31, 2012	Oct. 03, 2012	Dec. 31, 2011
Long-term debt
Long-Term Debt	$ 822,721,000	[1]		$ 822,721,000	[1]				$ 719,775,000	[1]
Long Term Debt (Textual) [Abstract]
Capitalized interest on our unproved properties	2,000,000		2,000,000	6,000,000		5,700,000
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	13,762,000		8,439,000	40,546,000		25,449,000
Net Cash Proceeeds from Debt Issuance								156,800,000
Long Term Debt (Additional Textual) [Abstract]
Lead bank's prime rate	3.25%			3.25%
Commitment Fee Basis Points On Unused Line of Credit Facility	50			50			50
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
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
Before Increase [Member]
Long Term Debt (Additional Textual) [Abstract]
Current Borrowing base	375,000,000			375,000,000
Current Committed borrowing base amount	300,000,000			300,000,000
After Increase [Member]
Long Term Debt (Additional Textual) [Abstract]
Current Borrowing base							450,000,000
Current Committed borrowing base amount							450,000,000
Bank Borrowings [Member]
Long Term Debt (Textual) [Abstract]
Percentage of applicable margin with federal fund rate	0.50%
Percentage of applicable margin with LIBOR	1.00%
Minimum Debt Instrument Escalating Basis Spread 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
Long Term Debt (Additional Textual) [Abstract]
Amount outstanding under credit facility	102,640,000			102,640,000					0
Bank Borrowings [Member] | Before Increase [Member]
Long Term Debt (Additional Textual) [Abstract]
Maximum availability aggregate facility amount							500,000,000
Bank Borrowings [Member] | After Increase [Member]
Long Term Debt (Additional Textual) [Abstract]
Number of syndicated banks							11
Senior notes due 2020 [Member]
Long-term debt
Long-Term Debt	222,076,000	[1]		222,076,000	[1]				221,873,000	[1]
Long Term Debt (Textual) [Abstract]
Senior notes, issued	225,000,000			225,000,000
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	5,200,000		5,100,000	15,500,000		15,400,000
Stated Rate of Senior notes	8.875%			8.875%
Percentage at which senior notes are issued, of par value	98.389%			98.389%
Effective Interest Rate On Senior Notes Including Discount	9.125%			9.125%
Initial Debt Issuance Costs Capitalized	5,000,000			5,000,000
Original unamortized issuance discount on senior notes	3,600,000			3,600,000
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%
Redemption of Debts with Net Proceeds of Qualifying Offerings at Price Above Principle Amount of Debts	108.875%
Holder right to require company to repurchase notes at purchase price in cash				101.00%
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]
Senior notes, issued	250,000,000			250,000,000
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	4,600,000		4,500,000	13,700,000		13,600,000
Stated Rate of Senior notes	7.125%			7.125%
Percentage at which senior notes are issued, of par value	100.00%			100.00%
Initial Debt Issuance Costs Capitalized	4,200,000			4,200,000
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 purchase price in cash				101.00%
Senior Notes Due 2022 [Member]
Long-term debt
Long-Term Debt	248,005,000	[1]		248,005,000	[1]				247,902,000	[1]
Long Term Debt (Textual) [Abstract]
Senior notes, issued	250,000,000			250,000,000
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	5,000,000			15,100,000
Stated Rate of Senior notes	7.875%			7.875%
Percentage at which senior notes are issued, of par value	99.156%			99.156%
Effective Interest Rate On Senior Notes Including Discount	8.00%			8.00%
Original unamortized issuance discount on senior notes	2,100,000			2,100,000
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%
Redemption of Debts with Net Proceeds of Qualifying Offerings at Price Above Principle Amount of Debts	107.875%
Holder right to require company to repurchase notes at purchase price in cash				101.00%
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	900,000		600,000	2,100,000		1,800,000
Long Term Debt (Additional Textual) [Abstract]
Commitment fees included in interest expense, net	300,000		400,000	1,100,000		1,100,000
Senior Notes Due Two Thousand Twenty Two [Member]
Long Term Debt (Textual) [Abstract]
Senior notes, issued	250,000,000			250,000,000
Initial Debt Issuance Costs Capitalized	4,800,000			4,800,000
Additional Senior Notes Due 2022 [Member]
Long Term Debt (Textual) [Abstract]
Senior notes, issued								150,000,000
Stated Rate of Senior notes								7.875%
Percentage at which senior notes are issued, of par value								105.00%
Effective Interest Rate On Senior Notes Including Discount								6.993%
Debt Instrument, Unamortized Premium								7,500,000
Initial Debt Issuance Costs Capitalized								$ 2,800,000

[1] Amounts are shown net of debt discount