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											12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Revolving Credit Facility		Sep. 06, 2013 Revolving Credit Facility	May 20, 2013 Revolving Credit Facility	Apr. 12, 2013 Revolving Credit Facility	Dec. 31, 2012 Revolving Credit Facility		Dec. 31, 2013 5% Senior Notes Due 2024 [Member]	May 20, 2013 5% Senior Notes Due 2024 [Member]	Dec. 31, 2013 6.50% Senior Notes Due 2023 [Member]	Jun. 28, 2012 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2013 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2012 6.50% Senior Notes Due 2021 [Member]	Nov. 09, 2011 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2013 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2012 6.625% Senior Notes Due 2019 [Member]	Feb. 07, 2011 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2013 3.50% Senior Convertible Notes	Dec. 31, 2012 3.50% Senior Convertible Notes	Dec. 31, 2010 3.50% Senior Convertible Notes	Dec. 31, 2013 Less than 25% Revolving Credit Facility	Dec. 31, 2013 More than 25% Less than 50% Revolving Credit Facility	Dec. 31, 2013 More than 50% Less than 75% Revolving Credit Facility	Dec. 31, 2013 More than 75% Less than 90% Revolving Credit Facility	Dec. 31, 2013 More than 90% Revolving Credit Facility	Dec. 31, 2013 Eurodollar Loans Revolving Credit Facility	Dec. 31, 2013 Eurodollar Loans Less than 25% Revolving Credit Facility	Dec. 31, 2013 Eurodollar Loans More than 25% Less than 50% Revolving Credit Facility	Dec. 31, 2013 Eurodollar Loans More than 50% Less than 75% Revolving Credit Facility	Dec. 31, 2013 Eurodollar Loans More than 75% Less than 90% Revolving Credit Facility	Dec. 31, 2013 Eurodollar Loans More than 90% Revolving Credit Facility	Dec. 31, 2013 ABR Loans or Swingline Loans Revolving Credit Facility	Dec. 31, 2013 ABR Loans or Swingline Loans Less than 25% Revolving Credit Facility	Dec. 31, 2013 ABR Loans or Swingline Loans More than 25% Less than 50% Revolving Credit Facility	Dec. 31, 2013 ABR Loans or Swingline Loans More than 50% Less than 75% Revolving Credit Facility	Dec. 31, 2013 ABR Loans or Swingline Loans More than 75% Less than 90% Revolving Credit Facility	Dec. 31, 2013 ABR Loans or Swingline Loans More than 90% Revolving Credit Facility	Dec. 31, 2013 2015 [Member] 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2013 2016 [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2013 2016 [Member] 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2013 2017 [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2013 2017 [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2013 2018 [Member] 5% Senior Notes Due 2024 [Member]	Dec. 31, 2013 2018 [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2013 2018 [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2013 2019 [Member] 5% Senior Notes Due 2024 [Member]	Dec. 31, 2013 2019 [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2013 2020 [Member] 5% Senior Notes Due 2024 [Member]	Dec. 31, 2013 2020 and Thereafter [Member] 6.50% Senior Notes Due 2023 [Member]	Dec. 31, 2013 2019 and Thereafter [Member] 6.50% Senior Notes Due 2021 [Member]	Dec. 31, 2013 2017 and Thereafter [Member] 6.625% Senior Notes Due 2019 [Member]	Dec. 31, 2013 2021 and Thereafter [Member] 5% Senior Notes Due 2024 [Member]	Feb. 12, 2014 Subsequent Event [Member] Revolving Credit Facility
Debt Instrument [Line Items]
Deferred financing fees				$ (3,400,000)								$ 9,800,000		$ 7,900,000			$ 6,900,000			$ 8,900,000
Maximum loan amount				2,500,000,000
Current borrowing base				1,300,000,000
Borrowing Base, Line of Credit						2,200,000,000	1,775,000,000	1,900,000,000
Reduction in Borrowing Base as a Percent of Senior Debt Prinicipal				25.00%
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%
Revolving credit facility	0	340,000,000		0					340,000,000																																															0
Letters of credit outstanding				800,000	[1]				800,000	[1]																																														800,000	[1]
Available Borrowing Capacity Under Credit Facility				1,299,200,000					659,200,000																																															1,299,200,000
6.625% Senior Notes, Due 2019	350,000,000	350,000,000
6.50% Senior Notes, Due 2021	350,000,000	350,000,000
6.50% Senior Notes, Due 2023	400,000,000	400,000,000
5% Senior Notes, Due 2024	500,000,000	0
Long-term Debt	1,600,000,000	1,100,000,000
Maximum amount of dividend payments in any given calendar year under the covenants of the debt instrument													6,500,000
Debt Instrument, Face Amount												500,000,000		400,000,000			350,000,000			350,000,000
Debt Instrument, Interest Rate, Stated Percentage												5.00%		6.50%			6.50%			6.625%	3.50%	3.50%	3.50%
Proceeds from Debt Issuance, Net of Issuance Costs, 5% Senior Notes											490,200,000
Net Proceeds from 6.50% Senior Notes due 2023													392,100,000
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)											105.00%		106.50%		106.50%
Debt Instrument, Call Feature											at a redemption price equal to 100 percent of the principal amount of the 2024 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 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%	102.50%	102.167%	101.083%	101.677%	101.083%	100.833%	100.00%	100.00%	100.00%	100.00%
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
Interest Costs, Capitalized During Period	$ 11,000,000	$ 12,100,000	$ 10,800,000

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