Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenues from services	$ 5,418.7	$ 5,464.8	$ 16,598.0	$ 15,396.8
Cost of services	4,528.1	4,564.2	13,899.3	12,846.7
Gross profit	890.6	900.6	2,698.7	2,550.1
Selling and administrative expenses	673.9	671.9	2,119.9	1,998.3
Operating profit	216.7	228.7	578.8	551.8
Interest and other expenses	11.8	11.7	38.4	38.5
Earnings before income taxes	204.9	217.0	540.4	513.3
Provision for income taxes	46.9	79.3	142.0	184.2
Net earnings	$ 158.0	$ 137.7	$ 398.4	$ 329.1
Net earnings per share - basic (in dollars per share)	$ 2.45	$ 2.06	$ 6.08	$ 4.89
Net earnings per share - diluted (in dollars per share)	$ 2.43	$ 2.04	$ 6.03	$ 4.84
Weighted average shares - basic (in shares)	64.5	66.8	65.6	67.3
Weighted average shares - diluted (in shares)	65.0	67.6	66.1	68.1