Debt (Details) (USD $) In Millions, unless otherwise specified			1 Months Ended							1 Months Ended			1 Months Ended			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Apr. 30, 2013 Interest rate swap contracts Designated as hedging instruments	Mar. 31, 2013 Interest rate swap contracts Designated as hedging instruments	Mar. 31, 2013 2.450% senior notes due 2015	Dec. 31, 2012 2.450% senior notes due 2015	Mar. 31, 2013 2012 issued notes Treasury rate locks	Mar. 31, 2013 1.900% senior notes due 2017	Dec. 31, 2012 1.900% senior notes due 2017	Apr. 30, 2013 1.900% senior notes due 2017 Interest rate swap contracts Designated as hedging instruments	Mar. 31, 2013 3.250% senior notes due 2022	Dec. 31, 2012 3.250% senior notes due 2022	Apr. 30, 2013 3.250% senior notes due 2022 Interest rate swap contracts Designated as hedging instruments	Mar. 31, 2013 3.950% senior notes due 2020	Dec. 31, 2012 3.950% senior notes due 2020	Mar. 31, 2013 3.950% senior notes due 2020 Interest rate swap contracts Designated as hedging instruments	Mar. 31, 2013 5.700% senior notes due 2040	Dec. 31, 2012 5.700% senior notes due 2040
Debt
Losses recorded to AOCI							$ 33.1
Carrying values of the entity's senior notes
Long-term debt	2,764.1	2,771.3			518.4	520.1		501.1	500.6		993.6	1,002.1		501.5	499.0		249.5	249.5
Senior notes, fair value	2,868
Interest rate (as a percent)					2.45%			1.90%			3.25%			3.95%			5.70%
Gains recorded as reduction of debt			3.8	26.0												2.5
Amount settled			$ 800.0							$ 400.0			$ 400.0			$ 400.0