Debt (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Debt Instrument [Line Items]
Total principal amount of debt	$ 3,410,300,000	$ 3,211,800,000
Less unamortized discount	(4,600,000)	(4,900,000)
Total long-term debt outstanding	3,405,700,000	3,206,900,000
Principal amount of senior notes outstanding	2,221,800,000
Debt Instrument, Maturity Date Range, Start	Sep. 01, 2016
Debt Instrument, Maturity Date Range, End	May 01, 2023
Senior Notes, stated interest rate, minimum (in hundredths)	5.25%
Senior Notes, stated interest rate, maximim (in hundredths)	6.875%
Revolving Credit Facility due 2016 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	888,500,000	690,000,000
Maturity date of long-term debt	Aug. 25, 2016
Borrowing capacity	1,500,000,000
Potential increased borrowing capacity	2,000,000,000
Number of maturity date extentions	2
Additional period with in which maturity to be extended	1 year
Weighted-average interest rate (in hundredths)	2.33%		2.05%
Amount outstanding	888,500,000
Letters of credit outstanding	3,700,000
Term Loan due 2017 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	300,000,000	300,000,000
Maturity date of long-term debt	Apr. 18, 2017
Additional period with in which maturity to be extended	1 year
Weighted-average interest rate (in hundredths)	2.23%		2.02%
6.05% 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
Maturity period of long-term debt	5 years
6.80% Senior Notes due 2018 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	134,000,000	134,000,000
6.80% Senior Notes due 2020 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	136,000,000	136,000,000
6.875% Senior Notes due 2021 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	625,000,000	625,000,000
5.375% Senior Notes due 2022 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	500,000,000	500,000,000
5.25% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	$ 650,000,000	$ 650,000,000