Long-Term Debt (Details) (USD $)	12 Months Ended						12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Oct. 28, 2013 Banks	Dec. 31, 2013 Minimum [Member] Eurodollar Interest Rate [Member]	Dec. 31, 2013 Minimum [Member] Alternative Base Interest Rate [Member]	Dec. 31, 2013 Maximum [Member] Eurodollar Interest Rate [Member]	Dec. 31, 2013 Maximum [Member] Alternative Base Interest Rate [Member]	Dec. 31, 2013 Line of Credit [Member]	Dec. 31, 2012 Line of Credit [Member]	Dec. 31, 2011 Line of Credit [Member]	Dec. 31, 2013 Senior notes due 2017 [Member]	Dec. 31, 2012 Senior notes due 2017 [Member]	Dec. 31, 2011 Senior notes due 2017 [Member]	Dec. 31, 2013 Senior notes due 2020 [Member]		Dec. 31, 2012 Senior notes due 2020 [Member]		Dec. 31, 2011 Senior notes due 2020 [Member]	Dec. 31, 2013 Senior Notes due 2022 [Member]		Dec. 31, 2012 Senior Notes due 2022 [Member]		Dec. 31, 2011 Senior Notes due 2022 [Member]	Dec. 31, 2013 Additional Senior Notes Due 2022 [Member]	Oct. 03, 2012 Additional Senior Notes Due 2022 [Member]
Bank Borrowings
Maturities on long-term debt											$ 265,000,000			$ 250,000,000			$ 225,000,000					$ 400,000,000
Capitalized interest on our unproved properties	7,200,000		7,900,000		7,700,000
Number of syndicated banks						11
Current Borrowing base						450,000,000
Maximum availability aggregate facility amount						500,000,000
Line of Credit, Current Commitment Amount	450,000,000
Long-Term Debt	1,142,368,000	[1]	916,934,000	[1]							265,000,000	39,400,000		250,000,000	250,000,000		222,446,000	[1]	222,147,000	[1]		404,922,000	[1]	405,387,000	[1]
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 eurodollar rate loans							150		250
Lead bank's prime rate	3.25%
Line of Credit Facility, Commitment Fee Percentage	0.50%
Maximum level of cash dividends in any fiscal year											15,000,000
Aggregate limitation on purchases of stock											50,000,000
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	69,382,000		57,303,000		35,566,000						6,000,000	3,700,000	2,400,000	18,300,000	18,200,000	18,200,000	20,700,000		20,600,000		20,600,000	31,600,000		22,400,000		1,700,000
Commitment fees included in interest expense, net											1,100,000	1,400,000	1,500,000
Senior Notes
Senior notes, issued														250,000,000			225,000,000					400,000,000		250,000,000				150,000,000
Stated rate of senior notes														7.125%			8.875%					7.875%
Percentage at which senior notes are issued, of par value														100.00%			98.389%							99.156%				105.00%
Effective interest rate on senior notes including discount																	9.125%							8.00%				6.993%
Original unamortized issuance discount on senior notes																	3,600,000					2,100,000		2,100,000
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			5,000,000					7,500,000
Holder right to require company to repurchase notes at purchase price in cash														101.00%			101.00%					101.00%
Interest expense including commitment fees and amortization of debt issuance costs relating to the credit facility	$ 69,382,000		$ 57,303,000		$ 35,566,000						$ 6,000,000	$ 3,700,000	$ 2,400,000	$ 18,300,000	$ 18,200,000	$ 18,200,000	$ 20,700,000		$ 20,600,000		$ 20,600,000	$ 31,600,000		$ 22,400,000		$ 1,700,000

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