Indebtedness (Details Textuals) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended			6 Months Ended					6 Months Ended													1 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Senior Subordinated Notes [Member]	Jun. 01, 2019 Senior Subordinated Notes [Member]	Jun. 01, 2016 Senior Subordinated Notes [Member]	Jun. 30, 2011 6.375% Senior Subordinated Notes due 2015 [Member]	Jun. 24, 2011 6.375% Senior Subordinated Notes due 2015 [Member]	May 25, 2011 6.375% Senior Subordinated Notes due 2015 [Member]	May 11, 2011 6.375% Senior Subordinated Notes due 2015 [Member]	Dec. 31, 2010 6.375% Senior Subordinated Notes due 2015 [Member]	Jun. 30, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	Jun. 24, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	May 25, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	May 11, 2011 7.5% senior subordinated notes due 2016, net of discount [Member]	Dec. 31, 2010 7.5% senior subordinated notes due 2016, net of discount [Member]	Jun. 30, 2011 7.5% Senior Subordinated Notes due 2017 [Member]	Dec. 31, 2010 7.5% Senior Subordinated Notes due 2017 [Member]	Jun. 30, 2011 7.25% Senior Subordinated Notes due 2018 [Member]	Dec. 31, 2010 7.25% Senior Subordinated Notes due 2018 [Member]	Jun. 30, 2011 8.0% senior subordinated notes due 2019, net of discount [Member]	Dec. 31, 2010 8.0% senior subordinated notes due 2019, net of discount [Member]	Jun. 30, 2011 6.75% Senior Subordinated Notes due 2020 [Member]	Dec. 31, 2010 6.75% Senior Subordinated Notes due 2020 [Member]	May 31, 2011 5.75% Senior Subordinated Notes due 2021 [Member]	Jun. 30, 2011 5.75% Senior Subordinated Notes due 2021 [Member]	Jun. 30, 2011 Maximum [Member]	Jun. 30, 2011 Minimum [Member]
Debt Instrument [Line Items]
Interest Rate on Senior Subordinated Notes									6.375%				6.375%	7.50%				7.50%	7.50%	7.50%	7.25%	7.25%	8.00%	8.00%	6.75%	6.75%		5.75%
Aggregate principal amount of senior subordinated notes												$ 150,000,000					$ 250,000,000										$ 500,000,000
Net proceeds after underwriting discounts and commissions			500,000,000																								491,300,000
Maximum redemption price of 5.75% notes as a percentage of principal amount						105.75%	100.00%	102.875%
Maximum percentage of the original aggregate amount of the 5.75% notes that can be redeemed						35.00%
Minimum percentage of original aggregate principal amount of 5.75% percentage 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 used to repay outstanding borrowings under credit facility			764,826,000	220,000,000
Repayment of subordinate debt			413,332,000						112,900,000					207,100,000
Purchase price of senior subordinate notes as a percentage of principal										102.125%	102.375%				103.75%	104.00%
Purchase price of senior subordinate notes										41,100,000	108,900,000				51,200,000	198,800,000
Maturity date of loan			Feb. 01, 2016
Line of Credit Facility [Line Items]
Annual rate of commitment fee paid on the undrawn balance																													0.375%	0.50%
Spread for Alternate Base Rate																													1.50%	0.50%
Spread for Libor Rate																													2.50%	1.50%
Indebtedness (Textuals) [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 Amount	1,500,000,000		1,500,000,000
Number of commercial banks included in current bank group	27		27
Percentage holding of commercial banks, Maximum	7.00%		7.00%
Notice period for increase in facility amount			20 days
Outstanding balance under the bank credit facility	0		0		274,000,000
Undrawn letters of credit	13,900,000		13,900,000
Borrowing capacity available under the facility amount	1,500,000,000		1,500,000,000
Weighted average interest rate on the bank credit facility	2.00%	2.20%	2.20%	2.20%
Commitment Fee Rate			0.375%
Ratio of debt to EBITDAX	4.25		4.25
Current ratio	1.0		1.0
Loss on early extinguishment of debt	$ (18,580,000)		$ (18,580,000)