Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues from services	$ 5,393.2	$ 5,418.7	$ 5,656.9	$ 5,522.4	$ 5,637.5	$ 5,464.8	$ 5,174.8	$ 4,757.2	$ 21,991.2	$ 21,034.3	$ 19,654.1
Gross profit	880.3	890.6	922.7	885.4	934.5	900.6	861.7	787.8	3,579.0	3,484.6	3,333.8
Operating profit	217.9	216.7	208.3	153.8	237.4	228.7	195.2	127.9	796.7	789.2	745.5
Net earnings	$ 158.3	$ 158.0	$ 143.4	$ 97.0	$ 216.3	$ 137.7	$ 117.0	$ 74.4	$ 556.7	$ 545.4	$ 443.7
Net earnings per share - basic (in dollars per share)	$ 2.56	$ 2.45	$ 2.18	$ 1.46	$ 3.26	$ 2.06	$ 1.74	$ 1.10	$ 8.62	$ 8.13	$ 6.33
Net earnings per share - diluted (in dollars per share)	2.54	2.43	2.17	1.45	3.22	2.04	1.72	1.09	8.56	8.04	6.27
Dividends per share (in dollars per share)	$ 1.01	$ 0	$ 1.01	$ 0	$ 0.93	$ 0	$ 0.93	$ 0	$ 2.02	$ 1.86	$ 1.72
Restructuring costs			$ 15.3	$ 24.0			$ 10.5	$ 24.0
Restructuring costs per diluted share (in dollars per share)			$ (0.18)	$ (0.27)			$ (0.10)	$ (0.30)
Net tax expense (benefits) related to the Tax Act and French tax reform					$ (73.7)				$ 3.2	$ (73.7)	$ 0.0
Net tax benefits per diluted share (in dollars per share)					$ 1.10