Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues from services	$ 5,277.1	$ 3,742.2	$ 10,201.5	$ 8,361.3
Cost of services	4,417.0	3,165.5	8,573.3	7,060.6
Gross profit	860.1	576.7	1,628.2	1,300.7
Selling and administrative expenses, excluding goodwill impairment charge	690.2	559.9	1,359.9	1,246.2
Goodwill impairment charges		66.8		66.8
Selling and administrative expenses	690.2	626.7	1,359.9	1,313.0
Operating profit (loss)	169.9	(50.0)	268.3	(12.3)
Interest and other expenses, net	2.8	5.8	8.2	26.3
Earnings (loss) before income taxes	167.1	(55.8)	260.1	(38.6)
Provision for income taxes	55.5	8.6	86.5	24.1
Net earnings (loss)	$ 111.6	$ (64.4)	$ 173.6	$ (62.7)
Net earnings (loss) per share - basic	$ 2.05	$ (1.11)	$ 3.17	$ (1.07)
Net earnings (loss) per share - diluted	$ 2.02	$ (1.11)	$ 3.13	$ (1.07)
Weighted average shares - basic (in shares)	54.5	58.2	54.8	58.5
Weighted average shares - diluted (in shares)	55.4	58.2	55.5	58.5