Long-Term Debt (Details) (USD $)	6 Months Ended			6 Months Ended						6 Months Ended						6 Months Ended					1 Months Ended	6 Months Ended	12 Months Ended		24 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011 Revolving Credit Facility	Dec. 31, 2010 Revolving Credit Facility	Jun. 30, 2011 Revolving Credit Facility Eurodollar Loans	Jun. 30, 2011 Revolving Credit Facility Eurodollar Loans Less than 25%	Jun. 30, 2011 Revolving Credit Facility Eurodollar Loans More than 25% Less than 50%	Jun. 30, 2011 Revolving Credit Facility Eurodollar Loans More than 50% Less than 75%	Jun. 30, 2011 Revolving Credit Facility Eurodollar Loans More than 75% Less than 90%	Jun. 30, 2011 Revolving Credit Facility Eurodollar Loans More than 90%	Jun. 30, 2011 Revolving Credit Facility ABR Loans or Swingline Loans	Jun. 30, 2011 Revolving Credit Facility ABR Loans or Swingline Loans Less than 25%	Jun. 30, 2011 Revolving Credit Facility ABR Loans or Swingline Loans More than 25% Less than 50%	Jun. 30, 2011 Revolving Credit Facility ABR Loans or Swingline Loans More than 50% Less than 75%	Jun. 30, 2011 Revolving Credit Facility ABR Loans or Swingline Loans More than 75% Less than 90%	Jun. 30, 2011 Revolving Credit Facility ABR Loans or Swingline Loans More than 90%	Jun. 30, 2011 Revolving Credit Facility Less than 25%	Jun. 30, 2011 Revolving Credit Facility More than 25% Less than 50%	Jun. 30, 2011 Revolving Credit Facility More than 50% Less than 75%	Jun. 30, 2011 Revolving Credit Facility More than 75% Less than 90%	Jun. 30, 2011 Revolving Credit Facility More than 90%	Feb. 28, 2011 6.625% Senior Notes due 2019	Jun. 30, 2011 6.625% Senior Notes due 2019	Feb. 14, 2017 6.625% Senior Notes due 2019	Feb. 14, 2016 6.625% Senior Notes due 2019	Feb. 14, 2019 6.625% Senior Notes due 2019	Feb. 07, 2011 6.625% Senior Notes due 2019	Dec. 31, 2010 6.625% Senior Notes due 2019	Mar. 31, 2011 3.50% Senior Convertible Notes due 2027	Jun. 30, 2011 3.50% Senior Convertible Notes due 2027	Dec. 31, 2010 3.50% Senior Convertible Notes due 2027	Apr. 04, 2007 3.50% Senior Convertible Notes due 2027
Long-term debt
Deferred financing fees		$ 8,500,000																								$ 8,600,000
Maximum loan amount		2,500,000,000
Aggregate commitment amount		1,000,000,000	678,000,000
Current borrowing base		1,300,000,000
Maximum dividend restriction amount		50,000,000
Variable interest rate basis				London Interbank Offered Rate						Prime
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%
Outstanding amount			48,000,000
Available borrowing capacity		999,400,000	629,500,000
Number of letters of credit outstanding		2
Letters of credit outstanding		608,000	483,000
Aggregate principal amount																										350,000,000					287,500,000
Interest rate on notes (as a percent)																						6.625%					6.625%		3.50%	3.50%
Target ratio of closing share price to conversion price as a condition for conversion or redemption of 3.50% Senior Convertible Notes (as a percent)																													130.00%
Common stock conversion price (in dollars per share)																													$ 54.42
Net proceeds from 6.625% Senior Notes	341,435,000																				341,400,000
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%
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)																						106.625%
Redemption price of debt instrument (as a percent)																							101.656%	103.313%	100.00%
Maximum amount of dividend payments in any given calendar year under the covenants of the debt instrument																						$ 6,500,000
Debt maturity period (in years)																						8
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be redeemable (in days)																												20	20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be redeemable (in days)																												30	30
Conversion trigger price of common stock (in dollars per share)																												$ 70.75
Maximum additional interest rate in case of failure to complete exchange offer or shelf registration statement is not declared effective on or before February 7, 2012 (as a percent)																						1.00%