Indebtedness (Details Textual) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended			6 Months Ended												3 Months Ended	6 Months Ended
	Apr. 30, 2012 CommercialBank	Jun. 30, 2012 CommercialBank	Jun. 30, 2011	Jun. 30, 2012 CommercialBank	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Maximum [Member]	Jun. 30, 2012 Minimum [Member]	Jun. 30, 2012 7.50% Senior Subordinated notes due 2017 [Member]	Dec. 31, 2011 7.50% Senior Subordinated notes due 2017 [Member]	Jun. 30, 2012 7.25% Senior Subordinated notes due 2018 [Member]	Dec. 31, 2011 7.25% Senior Subordinated notes due 2018 [Member]	Jun. 30, 2012 8.00% senior subordinated notes due 2019, net of discount [Member]	Dec. 31, 2011 8.00% senior subordinated notes due 2019, net of discount [Member]	Jun. 30, 2012 6.75% Senior Subordinated notes due 2020 [Member]	Dec. 31, 2011 6.75% Senior Subordinated notes due 2020 [Member]	Jun. 30, 2012 5.75% Senior Subordinated notes due 2021 [Member]	Dec. 31, 2011 5.75% Senior Subordinated notes due 2021 [Member]	Mar. 31, 2012 5.00% Senior Subordinated notes due 2022 [Member]	Jun. 30, 2012 5.00% Senior Subordinated notes due 2022 [Member]	Feb. 15, 2020 5.00% Senior Subordinated notes due 2022 [Member]	Feb. 15, 2017 5.00% Senior Subordinated notes due 2022 [Member]
Debt Instrument [Line Items]
Discount on Senior Subordinated notes													$ 11,438,000	$ 12,033,000					$ 10,500,000
Interest rate on senior subordinated notes									7.50%	7.50%	7.25%	7.25%	8.00%	8.00%	6.75%	6.75%	5.75%	5.75%		5.00%
Aggregate principal amount of senior subordinated notes																			600,000,000
Net proceeds after underwriting discounts and commissions				600,000,000	500,000,000														589,500,000
Maximum redemption price of notes as percentage of principal amount																				105.00%	100.00%	102.50%
Maximum percentage of original aggregate amount of notes that can be redeemed																				35.00%
Minimum percentage of original aggregate principal amount of notes to remain outstanding immediately after redemption																				65.00%
Maximum number of days after the date of the closing of the equity offering that the redemption should occur																				60 days
Proceeds to pay down our outstanding credit facility balance				649,000,000	764,826,000														350,000,000
Purchase price of senior subordinate notes as a percentage of principal		101.00%		101.00%
Line of Credit Facility [Line Items]
Spread for Alternate Base Rate							1.50%	0.50%
Spread for LIBOR Rate							2.50%	1.50%
Annual rate of commitment fee paid on the undrawn balance							0.50%	0.375%
Ratio of debt to EBITDAX							4.25	1.0
Current ratio							1.0	1.0
Indebtedness (Textual) [Abstract]
Interest costs, capitalized during period		0	0	0	0
Bank Credit facility, Borrowing base		2,000,000,000		2,000,000,000
Bank Credit facility, Facility amount		1,750,000,000		1,750,000,000
Number of commercial banks included in current bank group		28		28
Number of commercial banks added to bank group	3
Percentage holding of commercial banks, Maximum		9.00%		9.00%
Notice period for increase in facility amount				20 days
Outstanding balance under the bank credit facility		235,000,000		235,000,000		187,000,000
Undrawn letters of credit		62,100,000		62,100,000
Maturity date of loan				Feb. 18, 2016
Borrowing capacity available under the facility amount		$ 1,500,000,000		$ 1,500,000,000
Weighted average interest rate on the bank credit facility		2.70%	2.00%	2.30%	2.20%
Commitment fee		0.375%
Interest rate margin on LIBOR loans		1.50%		1.50%
Interest rate margin on base rate loans		0.50%		0.50%
Bank debt percentage		2.20%		2.20%		2.20%