Long-Term Debt (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Apr. 26, 2013 Banks	Dec. 31, 2012
Long-term debt
Long-Term Debt	$ 1,037,435,000	[1]		$ 1,037,435,000	[1]			$ 916,934,000	[1]
Long Term Debt (Textual) [Abstract]
Capitalized interest on our unproved properties	1,900,000		2,000,000	3,800,000		4,000,000
Maximum availability aggregate facility amount							500,000,000
Number of syndicated banks							11
Current Borrowing base							450,000,000
Line of Credit, Current Commitment Amount							450,000,000
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
Lead bank's prime rate	3.25%			3.25%
Commitment Fee Basis Points For The Credit Facility	0.375%
Maximum level of cash dividends in any fiscal year	15,000,000			15,000,000
Equity Restrictions	50,000,000			50,000,000
Interest expense including amortization of debt issuance costs	17,000,000		13,319,000	33,802,000		26,784,000
Bank Borrowings [Member]
Long Term Debt (Textual) [Abstract]
Amount outstanding under credit facility	160,000,000			160,000,000				39,400,000
Senior notes due 2020 [Member]
Long-term debt
Long-Term Debt	222,293,000	[1]		222,293,000	[1]			222,147,000	[1]
Long Term Debt (Textual) [Abstract]
Senior notes, issued	225,000,000			225,000,000
Interest expense including amortization of debt issuance costs	5,200,000		5,200,000	10,300,000		10,300,000
Stated Rate of Senior notes	8.875%			8.875%
Original unamortized issuance discount on senior notes	3,600,000			3,600,000
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%
Redemption price plus accrued and unpaid interest	104.438%
Redemption price, plus accrued and unpaid interest declining in twelve-month intervals	100.00%
Initial Debt Issuance Costs Capitalized	5,000,000			5,000,000
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 amortization of debt issuance costs	4,600,000		4,600,000	9,100,000		9,100,000
Stated Rate of Senior notes	7.125%			7.125%
Percentage at which senior notes are issued, of par value	100.00%			100.00%
Redemption price plus accrued and unpaid interest	102.375%
Redemption price, plus accrued and unpaid interest declining in twelve-month intervals	100.00%
Initial Debt Issuance Costs Capitalized	4,200,000			4,200,000
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	405,142,000	[1]		405,142,000	[1]			405,387,000	[1]
Long Term Debt (Textual) [Abstract]
Senior notes, issued	400,000,000			400,000,000
Interest expense including amortization of debt issuance costs	7,900,000		5,000,000	15,800,000		10,100,000
Stated Rate of Senior notes	7.875%			7.875%
Original unamortized issuance discount on senior notes	2,100,000			2,100,000
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%
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 amortization of debt issuance costs	1,200,000		600,000	2,400,000		1,200,000
Commitment fees included in interest expense, net	300,000		400,000	600,000		700,000
Original 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	7,500,000			7,500,000
Additional Senior Notes Due 2022 [Member]
Long Term Debt (Textual) [Abstract]
Senior notes, issued	150,000,000			150,000,000
Percentage at which senior notes are issued, of par value	105.00%			105.00%
Effective Interest Rate On Senior Notes Including Discount	6.993%			6.993%
Debt Instrument, Unamortized Premium	$ 7,500,000			$ 7,500,000

[1] Amounts are shown net of any debt discount or premium