Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Taxes
Provision for income taxes	$ (78.1)	$ (83.4)	$ (212.7)	$ (222.6)
Effective tax rate (as a percent)	21.80%	26.90%	21.80%	27.20%
Impact of acquisition-related expenses on the effective tax rate (in basis points)		0.40%		0.70%
Liability for unrecognized tax benefits, including penalties and interest, which if recognized would impact the effective tax rate	$ 136.0		$ 136.0
Unrecognized tax benefits that could change in the next twelve-month period	5.1		5.1
Impact of ASU 2016-09
Income Taxes
Provision for income taxes	$ 16.6		$ 45.8
Impact of tax benefits due to accounting standard for stock-based compensation (in basis points)	4.70%		4.70%