Debt (Details) (USD $)	12 Months Ended					12 Months Ended		12 Months Ended								12 Months Ended				3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Revolving Credit Facility due 2016 [Member]	Dec. 31, 2011 Revolving Credit Facility due 2016 [Member]	Dec. 31, 2012 Term Loan due 2017 [Member]	Dec. 31, 2011 Term Loan due 2017 [Member]	Dec. 31, 2012 6.05% Senior Notes due 2016 [Member]	Dec. 31, 2011 6.05% Senior Notes due 2016 [Member]	Dec. 31, 2012 6.80% Senior Notes due 2018 [Member]	Dec. 31, 2011 6.80% Senior Notes due 2018 [Member]	Dec. 31, 2012 6.80% Senior Notes due 2020 [Member]	Dec. 31, 2011 6.80% Senior Notes due 2020 [Member]	Dec. 31, 2012 6.875% Senior Notes due 2021 [Member]	Dec. 31, 2011 6.875% Senior Notes due 2021 [Member]	Dec. 31, 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]	Dec. 31, 2012 Senior Notes [Member]	Sep. 30, 2012 5.25% Senior Notes Due 2023 [Member]	Dec. 31, 2012 5.25% Senior Notes Due 2023 [Member]	Dec. 31, 2011 5.25% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Total principal amount of debt	$ 3,211,800,000	$ 1,684,900,000		$ 690,000,000	$ 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			$ 650,000,000	$ 0
Less unamortized discount	(4,900,000)	(5,500,000)
Total long-term debt outstanding	3,206,900,000	1,679,400,000
Long-term debt, interest rate (in hundredths)								6.05%								5.375%					5.25%
Maturity date of long-term debt				Aug. 25, 2016		Apr. 18, 2017		Sep. 01, 2016													May 01, 2023
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		1 year
Weighted-average interest rate (in hundredths)				2.08%		2.05%
Debt Instrument, Face Amount																					650,000,000
Proceeds from Issuance of Long-term Debt	1,450,000,000	0	1,034,400,000																	641,000,000
Amount outstanding				690,000,000
Letters of credit outstanding				4,100,000
Proceeds from issuance of debt																493,100,000
Debt issuance costs																6,900,000					9,000,000
Senior notes repurchased																			6,700,000
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
Non-cash loss on early extinguishment of debt	$ 600,000	$ 700,000	$ 13,300,000
Senior Notes, stated interest rate, minimum (in hundredths)	5.25%
Senior Notes, stated interest rate, maximim (in hundredths)	6.875%