Long-Term Debt (Details) $ in Thousands							3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($)	May. 01, 2015 USD ($) Banks	Oct. 03, 2012 USD ($)	Nov. 25, 2009 USD ($)	Jun. 01, 2007 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Oct. 27, 2014 USD ($)	Nov. 30, 2011 USD ($)
Bank Borrowings
Long-Term Debt	[1]	$ 1,149,165					$ 1,149,165		$ 1,149,165		$ 1,074,534
Capitalized interest on our unproved properties							1,200	$ 1,200	2,400	$ 2,500
Interest expense including amortization of debt issuance costs							18,741	18,649	36,969	37,098
Senior Notes [Member] | Senior Notes Due 2017 [Member]
Bank Borrowings
Long-Term Debt		250,000					250,000		250,000		250,000
Long-term Debt, Maturities, Repayments of Principal in Year Three		250,000					250,000		250,000
Interest expense including amortization of debt issuance costs							4,600	4,600	9,200	9,100
Senior Notes
Senior notes, issued						$ 250,000
Debt Instrument, Interest Rate, Stated Percentage						7.125%
Percentage at which senior notes are issued, of par value						100.00%
Debt Issuance Cost		1,000				$ 4,200
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	[1]	222,951					222,951		222,951		222,775
Long-term Debt, Maturities, Repayments of Principal after Year Five		225,000					225,000		225,000
Interest expense including amortization of debt issuance costs							5,200	5,200	10,400	10,400
Senior Notes
Senior notes, issued					$ 225,000
Debt Instrument, Interest Rate, Stated Percentage					8.875%
Original unamortized issuance discount on senior notes					$ 3,600
Percentage at which senior notes are issued, of par value					98.389%
Effective Interest Rate On Senior Notes Including Discount					9.125%
Debt Issuance Cost		2,800			$ 5,000
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	[1]	404,214					404,214		404,214		404,459
Long-term Debt, Maturities, Repayments of Principal after Year Five		400,000					400,000		400,000
Interest expense including amortization of debt issuance costs							7,900	7,900	15,800	15,800
Senior Notes
Senior notes, issued				$ 400,000									$ 250,000
Debt Instrument, Interest Rate, Stated Percentage				7.875%
Original unamortized issuance discount on senior notes													$ 2,100
Percentage at which senior notes are issued, of par value													99.156%
Effective Interest Rate On Senior Notes Including Discount													8.00%
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%
Debt Issuance Cost		5,600		$ 7,500
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
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
Line of Credit [Member]
Bank Borrowings
Long-Term Debt		$ 272,000					$ 272,000		$ 272,000		$ 197,300
Number of syndicated banks | Banks			11
Line of Credit Facility, Current Borrowing Capacity			$ 375,000									$ 417,600
Line of Credit, Current Commitment Amount			$ 375,000									$ 417,600
Line of Credit, Covenant, Liquidity Requirement			15.00%
Line of Credit, Covenant, Senior Secured Leverage Ratio, Minimum									3.0
Line of Credit, Covenant, Future Senior Secured Leverage Ratio, Minimum		2.5
Line of Credit, Covenant, Senior Secured Leverage Ratio, Company Actual									1.2
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%
Commitment Fee Basis Points For The Credit Facility							0.50%
Maximum level of cash dividends in any fiscal year		$ 15,000					$ 15,000		$ 15,000
Equity Restrictions		$ 50,000					50,000		$ 50,000
Line of Credit, Covenant, Working Capital Ratio, Minimum		1.0
Debt Instrument, Covenant, Working Capital Ratio, Company Actual		1.6
Debt Instrument, Covenant, Interest Coverage Ratio, Minimum									1.50
Debt Instrument, Covenant, Interest Coverage Ratio, Company Actual									3.0
Debt Instrument, Covenant, Future Interest Coverage Ratio, Minimum		2.75
Line of Credit, Required Security Interest on Oil and Gas Properties									75.00%
Interest expense including amortization of debt issuance costs							2,200	2,100	$ 4,000	4,200
Commitment fees included in interest expense, net							$ 100	$ 200	$ 400	$ 300
Senior Notes
Debt Issuance Cost		$ 2,200	$ 600
Write off of Deferred Debt Issuance Cost			$ 200
Line of Credit [Member] | Minimum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate							75
Line of Credit [Member] | Minimum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt Instrument Escalating Rates for Eurodollar Rate Loans							175
Line of Credit [Member] | Maximum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate							175
Line of Credit [Member] | Maximum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt Instrument Escalating Rates for Eurodollar Rate Loans							275
Debt Instrument, Redemption, Period One [Member] | Senior Notes [Member] | Senior Notes Due 2017 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage						100.00%
Debt Instrument, Redemption, Period One [Member] | Senior Notes [Member] | Senior Notes Due 2020 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage					104.438%
Debt Instrument, Redemption, Period One [Member] | Senior Notes [Member] | Senior Notes Due 2022 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage				103.938%
Debt Instrument, Redemption, Period Two [Member] | Senior Notes [Member] | Senior Notes Due 2020 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage					100.00%
Debt Instrument, Redemption, Period Two [Member] | Senior Notes [Member] | Senior Notes Due 2022 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage				100.00%

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