Long-Term Debt (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013		Dec. 31, 2011
Bank Borrowings
Long-Term Debt	$ 1,079,269,000	[1]		$ 1,079,269,000	[1]		$ 1,142,368,000	[1]
Capitalized interest on our unproved properties	1,200,000		1,800,000	3,700,000		5,600,000
Number of syndicated banks	11			11
Line of Credit Facility, Current Borrowing Capacity	417,600,000			417,600,000
Maximum availability aggregate facility amount	500,000,000			500,000,000
Line of Credit, Current Commitment Amount	417,600,000			417,600,000
Percentage of applicable margin with federal fund rate	0.50%
Percentage of applicable margin with LIBOR	1.00%
Lead bank's prime rate	3.25%
Interest expense including amortization of debt issuance costs	18,197,000		17,495,000	55,295,000		51,297,000
Minimum [Member]
Bank Borrowings
Commitment Fee Basis Points For The Credit Facility	0.375%
Minimum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate	50
Minimum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt instrument escalating rates for eurodolllar rate loans	150
Maximum [Member]
Bank Borrowings
Commitment Fee Basis Points For The Credit Facility	0.50%
Maximum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate	150
Maximum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt instrument escalating rates for eurodolllar rate loans	250
Senior Notes [Member] | Senior Notes Due 2017 [Member]
Bank Borrowings
Long-Term Debt	250,000,000			250,000,000			250,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	250,000,000			250,000,000
Interest expense including amortization of debt issuance costs	4,600,000		4,600,000	13,700,000		13,700,000
Senior Notes
Senior notes, issued	250,000,000			250,000,000
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 [Member] | Senior Notes Due 2020 [Member]
Bank Borrowings
Long-Term Debt	222,691,000	[1]		222,691,000	[1]		222,446,000	[1]
Long-term Debt, Maturities, Repayments of Principal after Year Five	225,000,000			225,000,000
Interest expense including amortization of debt issuance costs	5,200,000		5,200,000	15,600,000		15,500,000
Senior Notes
Senior notes, issued	225,000,000			225,000,000
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 [Member] | Senior Notes Due 2022 [Member]
Bank Borrowings
Long-Term Debt	404,578,000	[1]		404,578,000	[1]		404,922,000	[1]
Long-term Debt, Maturities, Repayments of Principal after Year Five	400,000,000			400,000,000
Interest expense including amortization of debt issuance costs	7,900,000		7,900,000	23,700,000		23,700,000
Senior Notes
Senior notes, issued	400,000,000			400,000,000					250,000,000
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%
Initial Debt Issuance Costs Capitalized	7,500,000			7,500,000
Holder right to require company to repurchase notes at purchase price in cash	101.00%
Senior Notes [Member] | Additional Senior Notes Due 2022 [Member]
Senior Notes
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
Line of Credit [Member]
Bank Borrowings
Long-Term Debt	202,000,000			202,000,000			265,000,000
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	1,700,000		1,600,000	5,900,000		4,000,000
Commitment fees included in interest expense, net	$ 200,000		$ 200,000	$ 600,000		$ 900,000

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