Long-Term Debt (Details) (USD $)	12 Months Ended											12 Months Ended											12 Months Ended						12 Months Ended			12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Revolving Credit Facility		Feb. 14, 2013 Revolving Credit Facility		Aug. 01, 2012 Revolving Credit Facility	Dec. 31, 2011 Revolving Credit Facility		May 27, 2011 Revolving Credit Facility	Dec. 31, 2012 Revolving Credit Facility Less than 25%	Dec. 31, 2012 Revolving Credit Facility More than 25% Less than 50%	Dec. 31, 2012 Revolving Credit Facility More than 50% Less than 75%	Dec. 31, 2012 Revolving Credit Facility More than 75% Less than 90%	Dec. 31, 2012 Revolving Credit Facility More than 90%	Dec. 31, 2012 Revolving Credit Facility Eurodollar Loans	Dec. 31, 2012 Revolving Credit Facility Eurodollar Loans Less than 25%	Dec. 31, 2012 Revolving Credit Facility Eurodollar Loans More than 25% Less than 50%	Dec. 31, 2012 Revolving Credit Facility Eurodollar Loans More than 50% Less than 75%	Dec. 31, 2012 Revolving Credit Facility Eurodollar Loans More than 75% Less than 90%	Dec. 31, 2012 Revolving Credit Facility Eurodollar Loans More than 90%	Dec. 31, 2012 Revolving Credit Facility ABR Loans or Swingline Loans	Dec. 31, 2012 Revolving Credit Facility ABR Loans or Swingline Loans Less than 25%	Dec. 31, 2012 Revolving Credit Facility ABR Loans or Swingline Loans More than 25% Less than 50%	Dec. 31, 2012 Revolving Credit Facility ABR Loans or Swingline Loans More than 50% Less than 75%	Dec. 31, 2012 Revolving Credit Facility ABR Loans or Swingline Loans More than 75% Less than 90%	Dec. 31, 2012 Revolving Credit Facility ABR Loans or Swingline Loans More than 90%	Dec. 31, 2012 3.50% Senior Convertible Notes	Dec. 31, 2011 3.50% Senior Convertible Notes	Dec. 31, 2010 3.50% Senior Convertible Notes	Dec. 31, 2012 6.50% Senior Notes Due 2023 [Member]	Jun. 29, 2012 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2011 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2010 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2012 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2011 6.50% Senior Notes Due 2021 [Member]	Nov. 09, 2011 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2010 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2011 6.625% Senior Notes Due 2019 [Member]	Feb. 07, 2011 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2010 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2012 2015 [Member] 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2012 2016 [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 2016 [Member] 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2012 2017 [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2012 2017 [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 2018 [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2012 2018 [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 2019 [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2012 2020 and Thereafter [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2012 2019 and Thereafter [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 2017 and Thereafter [Member] 6.625% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Deferred financing fees				$ (8,700,000)																													$ 7,900,000					$ 6,900,000				$ 8,900,000
Maximum loan amount				2,500,000,000
Current borrowing base				1,000,000,000
Borrowing Base, Line of Credit								1,550,000,000			1,400,000,000
Maximum dividend restriction amount				50,000,000
Line of Credit Facility, Interest Rate Description																	London Interbank Offered Rate plus the applicable margin from the utilization table						Prime plus the applicable margin from the utilization table
Interest rate (as a percent)																		1.50%	1.75%	2.00%	2.25%	2.50%		0.50%	0.75%	1.00%	1.25%	1.50%
Commitment Fee Rate (as a percent)												0.375%	0.375%	0.50%	0.50%	0.50%
Long-term credit facility	340,000,000	0		340,000,000		407,500,000			0
Letters of credit outstanding				800,000	[1]	800,000	[1]		600,000	[1]
Available Borrowing Capacity Under Credit Facility				659,200,000		591,700,000			999,400,000
Debt Instrument, Interest Rate, Stated Percentage																													3.50%	3.50%	3.50%	6.50%	6.50%	0.00%	0.00%	6.50%	6.50%	6.50%	0.00%	6.625%	6.625%	6.625%	0.00%
Debt Conversion, Converted Instrument, Amount	2,641,000																												281,300,000
Debt instrument convertible conversion rate																													864,106
Repayments of Convertible Debt																													6,200,000
Aggregate principal amount																																	400,000,000					350,000,000				350,000,000
Net Proceeds from 6.50% Senior Notes due 2023	392,138,000	0	0
Net proceeds from debt issuance																																					343,100,000				341,100,000
Maximum percentage of the principal amount of the debt instrument which the entity may redeem with proceeds from equity offerings in the first through third year following the issuance year (as a percent)																																35.00%				35.00%				35.00%
Redemption price of debt instrument if redeemed with proceeds from qualified equity offerings in the first through third year following the issuance year (as a percent)																																106.50%				106.50%				106.625%
Debt Instrument, Call Feature																																redemption price equal to 100 percent of the principal amount of the 2023 Notes to be redeemed, plus a specified make-whole premium and accrued and unpaid interest to the applicable redemption date.				redemption price equal to 100% of the principal amount, plus a specified make-whole premium and accrued and unpaid interest				redemption price equal to 100% of the principal amount, plus a specified make-whole premium and accrued and unpaid interest
Early Debt Redemption, Redemption Price as a Percent of Face Amount																																												103.313%	103.25%	101.656%	103.25%	102.167%	102.167%	101.083%	101.083%	100.00%	100.00%	100.00%
Maximum amount of dividend payments in any given calendar year under the covenants of the debt instrument																																6,500,000				6,500,000				6,500,000
Interest Costs, Capitalized During Period	$ 12,100,000	$ 10,800,000	$ 4,300,000

[1] Letters of credit reduce the amount available under the credit facility on a dollar-for-dollar basis.