Debt (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended		6 Months Ended		6 Months Ended								6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Revolving Credit Facility due 2016 [Member]	Dec. 31, 2011 Revolving Credit Facility due 2016 [Member]	Jun. 30, 2012 Term Loan due 2017 [Member]	Dec. 31, 2011 Term Loan due 2017 [Member]	Jun. 30, 2012 6.05% Senior Notes due 2016 [Member]	Dec. 31, 2011 6.05% Senior Notes due 2016 [Member]	Jun. 30, 2012 6.80% Senior Notes due 2018 [Member]	Dec. 31, 2011 6.80% Senior Notes due 2018 [Member]	Jun. 30, 2012 6.80% Senior Notes due 2020 [Member]	Dec. 31, 2011 6.80% Senior Notes due 2020 [Member]	Jun. 30, 2012 6.875% Senior Notes due 2021 [Member]	Dec. 31, 2011 6.875% Senior Notes due 2021 [Member]	Jun. 30, 2012 5.375% Senior Notes due 2022 [Member]	Mar. 31, 2012 5.375% Senior Notes due 2022 [Member]	Dec. 31, 2011 5.375% Senior Notes due 2022 [Member]	Jun. 30, 2012 Senior Notes [Member]
Debt Instrument [Line Items]
Total principal amount of debt	$ 1,871,800,000		$ 1,871,800,000		$ 1,684,900,000	$ 0	$ 606,500,000	$ 300,000,000	$ 0	$ 176,800,000	$ 176,800,000	$ 134,000,000	$ 138,600,000	$ 136,000,000	$ 138,000,000	$ 625,000,000	$ 625,000,000	$ 500,000,000	$ 500,000,000	$ 0
Less unamortized discount	(5,200,000)		(5,200,000)		(5,500,000)
Total long-term debt outstanding	1,866,600,000		1,866,600,000		1,679,400,000
Long-term debt, interest rate (in hundredths)										6.05%
Maturity date of long-term debt						Aug. 25, 2016		Apr. 18, 2017		Sep. 01, 2016
Maturity period of long-term debt										5 years
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)						2.05%		2.02%
Amount outstanding						0
Letters of credit outstanding						4,100,000
Proceeds from issuance of debt																		493,100,000
Debt issuance costs																			6,900,000
Senior notes repurchased																					6,700,000
Principal amount of senior notes outstanding	1,571,800,000		1,571,800,000
Non-cash loss on early extinguishment of debt	$ 600,000	$ 0	$ 600,000	$ 0
Senior Notes, stated interest rate, minimum (in hundredths)			5.375%
Senior Notes, stated interest rate, maximim (in hundredths)			6.875%