Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Total principal amount of debt	$ 1,678,900,000	$ 1,684,900,000
Less unamortized discount	(5,400,000)	(5,500,000)
Total long-term debt outstanding	1,673,500,000	1,679,400,000
Principal amount of senior notes outstanding	1,578,400,000
Senior Notes, stated interest rate, minimum (in hundredths)	5.375%
Senior Notes, stated interest rate, maximim (in hundredths)	6.875%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total principal amount of debt	100,500,000	606,500,000
Maturity date of long-term debt	Aug. 25, 2016
Borrowing capacity	1,500,000,000
Potential increased borrowing capacity	2,000,000,000
Weighted-average interest rate (in hundredths)	2.06%
Number of maturity date extentions	2
Amount outstanding	100,500,000
Letters of credit outstanding	4,100,000
Senior Notes Due 2016 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	176,800,000	176,800,000
Long-term debt, interest rate (in hundredths)	6.05%
Maturity date of long-term debt	Sep. 01, 2016
Senior Notes Due 2018 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	138,600,000	138,600,000
Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	138,000,000	138,000,000
Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	625,000,000	625,000,000
Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	500,000,000	0
Long-term debt, interest rate (in hundredths)	5.375%
Proceeds from issuance of debt	493,000,000
Debt issuance costs	$ 7,000,000