Consolidated Statements of Operations (Unaudited) - USD ($) shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues from services	$ 4,584,800,000	$ 5,248,900,000	$ 12,946,100,000	$ 15,666,900,000
Cost of services	3,859,700,000	4,408,600,000	10,920,300,000	13,151,400,000
Gross profit	725,100,000	840,300,000	2,025,800,000	2,515,500,000
Selling and administrative expenses, excluding goodwill impairment charges	663,500,000	623,300,000	1,909,700,000	1,998,200,000
Goodwill impairment charges	0		66,800,000	64,000,000.0
Selling and administrative expenses	663,500,000	623,300,000	1,976,500,000	2,062,200,000
Operating profit	61,600,000	217,000,000.0	49,300,000	453,300,000
Interest and other expenses (income), net	6,000,000.0	12,200,000	32,300,000	(46,100,000)
Earnings before income taxes	55,600,000	204,800,000	17,000,000.0	499,400,000
Provision for income taxes	45,300,000	58,700,000	69,400,000	172,500,000
Net earnings (loss)	$ 10,300,000	$ 146,100,000	$ (52,400,000)	$ 326,900,000
Net earnings (loss) per share – basic	$ 0.18	$ 2.44	$ (0.90)	$ 5.43
Net earnings (loss) per share – diluted	$ 0.18	$ 2.42	$ (0.90)	$ 5.40
Weighted average shares - basic (in shares)	58.2	59.9	58.4	60.2
Weighted average shares - diluted (in shares)	58.5	60.3	58.4	60.6