CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenues	$ 3,923.9	$ 3,399.8	$ 2,626.0
Cost of services	2,793.9	2,376.1	1,775.9
Gross profit	1,130.0	1,023.7	850.1
Selling, general and administrative expenses	770.3	705.0	592.0
Amortization of intangible assets	51.1	58.5	33.4
Operating income	308.6	260.2	224.7
Interest expense	(52.9)	(56.0)	(27.6)
Write-off of loan costs	(18.9)	0.0	0.0
Income before income taxes	236.8	204.2	197.1
Provision for income taxes	62.0	46.2	39.2
Income from continuing operations	174.8	158.0	157.9
Loss from discontinued operations, net of income taxes	(0.1)	(0.3)	(0.2)
Net income	$ 174.7	$ 157.7	$ 157.7
Earnings per share:
Basic	$ 3.31	$ 3.02	$ 3.01
Diluted	$ 3.28	$ 2.98	$ 2.97
Number of shares and share equivalents used to calculate earnings per share:
Basic (in shares)	52.8	52.3	52.5
Diluted (in shares)	53.4	53.1	53.2
Reconciliation of net income to comprehensive income:
Net income	$ 174.7	$ 157.7	$ 157.7
Foreign currency translation adjustment	(0.7)	(2.7)	6.5
Comprehensive income	$ 174.0	$ 155.0	$ 164.2