Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenues from services	$ 5,464.8	$ 5,088.2	$ 15,396.8	$ 14,698.0
Cost of services	4,564.2	4,229.9	12,846.7	12,205.2
Gross profit	900.6	858.3	2,550.1	2,492.8
Selling and administrative expenses	672.7	647.2	2,000.6	1,954.0
Operating profit	227.9	211.1	549.5	538.8
Interest and other expenses	10.9	11.4	36.2	34.4
Earnings before income taxes	217.0	199.7	513.3	504.4
Provision for income taxes	79.3	70.5	184.2	188.1
Net earnings	$ 137.7	$ 129.2	$ 329.1	$ 316.3
Net earnings per share - basic (in dollars per share)	$ 2.06	$ 1.89	$ 4.89	$ 4.46
Net earnings per share - diluted (in dollars per share)	$ 2.04	$ 1.87	$ 4.84	$ 4.42
Weighted average shares - basic (in shares)	66.8	68.4	67.3	70.9
Weighted average shares - diluted (in shares)	67.6	69.0	68.1	71.6