Long-Term Debt (Details) (USD $)	3 Months Ended			0 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Mar. 31, 2015		Jun. 01, 2007	Nov. 25, 2009	Oct. 03, 2012	May 01, 2015	Dec. 31, 2014		Nov. 30, 2011	Oct. 27, 2014
Bank Borrowings
Long-Term Debt	$ 1,124,201,000	[1]		$ 1,124,201,000	[1]					$ 1,074,534,000	[1]
Capitalized interest on our unproved properties	1,200,000		1,300,000
Interest expense including amortization of debt issuance costs	18,228,000		18,449,000
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
Senior Notes
Senior notes, issued						250,000,000
Debt Instrument, Interest Rate, Stated Percentage						7.125%
Percentage at which senior notes are issued, of par value						100.00%
Debt Issuance Cost				1,200,000		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	222,863,000	[1]		222,863,000	[1]					222,775,000	[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
Senior Notes
Senior notes, issued							225,000,000
Debt Instrument, Interest Rate, Stated Percentage							8.875%
Original unamortized issuance discount on senior notes							3,600,000
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				3,000,000			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	404,338,000	[1]		404,338,000	[1]					404,459,000	[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
Senior Notes
Senior notes, issued								400,000,000				250,000,000
Debt Instrument, Interest Rate, Stated Percentage								7.875%
Original unamortized issuance discount on senior notes												2,100,000
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				5,800,000				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
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
Line of Credit [Member]
Bank Borrowings
Long-Term Debt	247,000,000			247,000,000						197,300,000
Line of Credit Facility, Current Borrowing Capacity													417,600,000
Line of Credit, Current Commitment Amount													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%
Maximum level of cash dividends in any fiscal year	15,000,000			15,000,000
Equity Restrictions	50,000,000			50,000,000
Line of Credit, Covenant, Working Capital Ratio, Minimum				1.0
Debt Instrument, Covenant, Working Capital Ratio, Company Actual				2.1
Debt Instrument, Covenant, Interest Coverage Ratio, Minimum				2.75
Debt Instrument, Covenant, Interest Coverage Ratio, Company Actual				4.1
Line of Credit, Required Security Interest on Oil and Gas Properties	75.00%
Interest expense including amortization of debt issuance costs	1,700,000		2,100,000
Commitment fees included in interest expense, net	200,000		200,000
Senior Notes
Debt Issuance Cost				2,100,000
Line of Credit [Member] | Minimum [Member]
Bank Borrowings
Commitment Fee Basis Points For The Credit Facility	0.375%
Line of Credit [Member] | Minimum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate	50
Line of Credit [Member] | Minimum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt Instrument Escalating Rates for Eurodollar Rate Loans	150
Line of Credit [Member] | Maximum [Member]
Bank Borrowings
Commitment Fee Basis Points For The Credit Facility	0.50%
Line of Credit [Member] | Maximum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate	150
Line of Credit [Member] | Maximum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt Instrument Escalating Rates for Eurodollar Rate Loans	250
Debt Instrument, Redemption, Period One [Member] | Senior Notes [Member] | Senior Notes Due 2017 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage						101.188%
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 2017 [Member]
Senior Notes
Debt Instrument, Redemption Price, Percentage						100.00%
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%
Subsequent Event [Member] | Line of Credit [Member]
Bank Borrowings
Number of syndicated banks									11
Line of Credit Facility, Current Borrowing Capacity									375,000,000
Line of Credit, Current Commitment Amount									$ 375,000,000
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
Percentage of applicable margin with federal fund rate									0.50%
Percentage of applicable margin with LIBOR									1.00%
Debt Instrument, Covenant, Interest Coverage Ratio, Minimum									1.5
Subsequent Event [Member] | Line of Credit [Member] | Minimum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate									75
Subsequent Event [Member] | Line of Credit [Member] | Minimum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt Instrument Escalating Rates for Eurodollar Rate Loans									175
Subsequent Event [Member] | Line of Credit [Member] | Maximum [Member] | Alternative Base Interest Rate [Member]
Bank Borrowings
Debt instrument escalating basis spread on base rate									175
Subsequent Event [Member] | Line of Credit [Member] | Maximum [Member] | Eurodollar Interest Rate [Member]
Bank Borrowings
Debt Instrument Escalating Rates for Eurodollar Rate Loans									275

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