Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Dec. 31, 2016
Income Tax [Line Items]
Income tax based on statutory rate		$ 158.5	$ 258.1		$ 245.5
Increase (decrease) resulting from:
Repatriation of non-United States earnings		2.5	69.7		(10.5)
State income taxes, net of federal benefit		8.2	1.1		2.2
Change in valuation allowance		0.7	(6.9)		(6.0)
Work Opportunity Tax Credit		(8.8)	(10.5)		(11.0)
Foreign-Derived Intangible Income deduction		(12.5)	0.0		0.0
United States Tax Act and French tax reform	$ (73.7)	3.2	(73.7)		0.0
Other, net		7.0	12.9		19.9
Total provision		198.0	191.9		257.6
French business tax, gross		74.8	72.1		63.1
French business tax
Increase (decrease) resulting from:
Non-United States tax rate difference		59.1	46.9		41.0
French CICE
Increase (decrease) resulting from:
Non-United States tax rate difference		(39.9)	(77.1)		0.0
French tax benefits related to the CICE					58.9
Other
Increase (decrease) resulting from:
Non-United States tax rate difference		20.0	(28.6)		(23.5)
Other | Benefits related to French earnings that are deemed to be permanently invested
Increase (decrease) resulting from:
Non-United States tax rate difference			(2.4)		(1.8)
French business tax difference
Increase (decrease) resulting from:
Non-United States tax rate difference		(10.1)	0.4		$ 0.4
Repatriation of non-United States earnings		$ (25.5)	$ 1.3
Expense recorded prior to the Tax Act
Increase (decrease) resulting from:
Repatriation of non-United States earnings				$ 83.3
United States
Income Tax Disclosure [Abstract]
U.S. Federal statutory rate (as a percent)		21.00%	35.00%		35.00%