Long-Term Debt (Details) (USD $)	3 Months Ended			6 Months Ended					3 Months Ended						6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended			6 Months Ended					3 Months Ended			6 Months Ended
	Jun. 30, 2014 Banks		Jun. 30, 2013	Jun. 30, 2014 Banks		Jun. 30, 2013	Dec. 31, 2013		Jun. 30, 2014 Minimum [Member] Eurodollar Interest Rate [Member]	Jun. 30, 2014 Minimum [Member] Alternative Base Interest Rate [Member]	Jun. 30, 2014 Maximum [Member] Eurodollar Interest Rate [Member]	Jun. 30, 2014 Maximum [Member] Alternative Base Interest Rate [Member]	Jun. 30, 2014 Line of Credit [Member]	Jun. 30, 2013 Line of Credit [Member]	Jun. 30, 2014 Line of Credit [Member]	Jun. 30, 2013 Line of Credit [Member]	Dec. 31, 2013 Line of Credit [Member]	Jun. 30, 2014 Senior notes due 2017 [Member]	Jun. 30, 2013 Senior notes due 2017 [Member]	Jun. 30, 2014 Senior notes due 2017 [Member]	Jun. 30, 2013 Senior notes due 2017 [Member]	Dec. 31, 2013 Senior notes due 2017 [Member]	Jun. 30, 2014 Senior notes due 2020 [Member]		Jun. 30, 2013 Senior notes due 2020 [Member]	Jun. 30, 2014 Senior notes due 2020 [Member]		Jun. 30, 2013 Senior notes due 2020 [Member]	Dec. 31, 2013 Senior notes due 2020 [Member]		Jun. 30, 2014 Senior Notes Due 2022 [Member]		Jun. 30, 2013 Senior Notes Due 2022 [Member]	Jun. 30, 2014 Senior Notes Due 2022 [Member]		Jun. 30, 2013 Senior Notes Due 2022 [Member]	Dec. 31, 2013 Senior Notes Due 2022 [Member]		Dec. 31, 2011 Senior Notes Due 2022 [Member]	Jun. 30, 2014 Additional Senior Notes Due 2022 [Member]	Jul. 15, 2014 Subsequent Event [Member]
Bank Borrowings
Long-term Debt, Maturities, Repayments of Principal in Year Three																		$ 250,000,000		$ 250,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five																							225,000,000			225,000,000					400,000,000			400,000,000
Capitalized interest on our unproved properties	1,200,000		1,900,000	2,500,000		3,800,000
Number of syndicated banks	11			11
Line of Credit Facility, Current Borrowing Capacity	450,000,000			450,000,000																																					417,600,000
Long-Term Debt	1,178,301,000	[1]		1,178,301,000	[1]		1,142,368,000	[1]					301,000,000		301,000,000		265,000,000	250,000,000		250,000,000		250,000,000	222,607,000	[1]		222,607,000	[1]		222,446,000	[1]	404,694,000	[1]		404,694,000	[1]		404,922,000	[1]
Maximum availability aggregate facility amount	500,000,000			500,000,000
Line of Credit, Current Commitment Amount	450,000,000			450,000,000																																					417,600,000
Percentage of applicable margin with federal fund rate	0.50%
Percentage of applicable margin with LIBOR	1.00%
Debt instrument escalating basis spread on base rate										50		150
Debt instrument escalating rates for eurodolllar rate loans									150		250
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													15,000,000		15,000,000
Equity Restrictions													50,000,000		50,000,000
Interest expense including amortization of debt issuance costs	18,649,000		17,000,000	37,098,000		33,802,000							2,100,000	1,200,000	4,200,000	2,400,000		4,600,000	4,600,000	9,100,000	9,100,000		5,200,000		5,200,000	10,400,000		10,300,000			7,900,000		7,900,000	15,800,000		15,800,000
Commitment fees included in interest expense, net													200,000	300,000	300,000	600,000
Senior Notes
Senior notes, issued																		250,000,000		250,000,000			225,000,000			225,000,000					400,000,000			400,000,000					250,000,000	150,000,000
Stated Rate of Senior notes																		7.125%		7.125%			8.875%			8.875%					7.875%			7.875%
Original unamortized issuance discount on senior notes																							3,600,000			3,600,000					2,100,000			2,100,000
Percentage at which senior notes are issued, of par value																		100.00%		100.00%			98.389%			98.389%					99.156%			99.156%						105.00%
Effective Interest Rate On Senior Notes Including Discount																							9.125%			9.125%					8.00%			8.00%						6.993%
Debt Instrument, Unamortized Premium																																								7,500,000
Redemption price plus accrued and unpaid interest																		102.375%					104.438%								103.938%
Redemption price, plus accrued and unpaid interest declining in twelve-month intervals																		100.00%					100.00%								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																		$ 4,200,000		$ 4,200,000			$ 5,000,000			$ 5,000,000					$ 7,500,000			$ 7,500,000
Holder right to require company to repurchase notes at purchase price in cash																		101.00%					101.00%								101.00%

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