Long - Term Debt (Details Textual) (USD $)	6 Months Ended		6 Months Ended						6 Months Ended						6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Bank Credit Facility [Member]	Aug. 02, 2012 Bank Credit Facility [Member]	Jun. 30, 2012 8.625 % Senior Notes due 2017 [Member]	Dec. 31, 2011 8.625 % Senior Notes due 2017 [Member]	Jun. 30, 2012 6.75 % Senior Subordinated Notes due 2014 [Member]	Dec. 31, 2011 6.75 % Senior Subordinated Notes due 2014 [Member]	Jun. 30, 2012 Bank debt [Member]	Aug. 02, 2012 Bank debt [Member]	Dec. 31, 2011 Bank debt [Member]	Apr. 26, 2011 Bank debt [Member]	Aug. 02, 2012 Letter of Credit [Member]	Jun. 30, 2012 Letter of Credit [Member]	Jun. 30, 2012 Senior Convertible Notes Due 2017 [Member]	Jun. 29, 2012 Senior Convertible Notes Due 2017 [Member]	Mar. 06, 2012 Senior Convertible Notes Due 2017 [Member]
Long Term Debt (Textual) [Abstract]
Credit facility initial date									Apr. 26, 2011
Redetermined base borrowing and Credit Facility			$ 400,000,000									$ 700,000,000
Bank credit facility maturing with Syndicated Bank Group									Sep. 15, 2014
Maturity of new credit facility if note issue under 2004 indenture are retired on or before April 15, 2014			Apr. 26, 2015						Apr. 15, 2014						Apr. 01, 2017
Initial bank and availability under facility									400,000,000	372,900,000
Aggregate Principal amount of Senior Subordinated Notes																	300,000,000
Outstanding borrowing under bank credit facility				0					0				27,100,000	27,100,000
Oil and gas reserve as a proportion of discounted present value of the future net cash flow, for Mortgage									80.00%
Proceeds from issuance of senior notes	300,000,000														291,100,000
Interest rate due on convertible note due 2017																	1.75%
Initial conversion rate of convertible note 2017															Initial conversion rate of 23.4449 shares of our common stock per $1,000 principal amount of 2017 Convertible Notes
Initial conversion price of convertible note 2017															$ 42.65
Closing share price																$ 25.34
Convertible notes principal amount															1,000
Percentage of current conversion price lower than closing sale price															130.00%
Maximum trading day															20 days
Consecutive trading days ending on last trading day															30 days
Period in which distribution is made to all holders of common stock rights, option or warrants															45 days
Consecutive trading days immediately preceding, but excluding, declaration date															10 days
Percentage in which distribution has per share value exceeding closing sale price of common stock															10.00%
Percentage in which closing sale price of common stock is excess of convertible notes															98.00%
Number of consecutive business day															5 days
Number of consecutive trading day															P5D
Number consecutive trading-day conversion period															25 days
Payment for call option															70,800,000
Strike price per share															$ 55.91
Percentage of convertible notes principal amount															100.00%
Anti-dilution adjustments for purchases of call option															7,033,470
Proceeds from sale of sold warrants															40,100,000
Carrying amount of liability component															229,200,000
Carrying amount of equity component															70,800,000
Transaction costs allocated to liability and equity components															8,900,000
Accrued interest	15,735,000	14,059,000
Long-term debt	$ 808,050,000	$ 620,000,000			$ 375,000,000	$ 375,000,000	$ 200,000,000	$ 200,000,000			$ 45,000,000				$ 233,100,000