Revenue - Impact of new revenue guidance on statement of operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	$ 2,607.5	$ 2,118.8	$ 2,121.7	$ 1,903.0	$ 2,401.9	$ 2,076.0	$ 1,974.3	$ 1,767.7	$ 8,751.0	$ 8,219.9	$ 6,923.9
Cost of services									6,981.7	6,642.4	5,639.8
Total costs and expenses									8,563.7	8,207.3	7,095.0
Operating income									187.3	12.6	(171.1)
Loss before income taxes									42.8	(210.8)	(341.8)
Benefit for income taxes									42.6	(25.0)	(120.5)
Net loss	$ 3.1	$ 11.7	$ 6.3	$ (20.9)	$ (18.0)	$ (41.4)	$ (33.5)	$ (92.9)	$ 0.2	(185.8)	$ (221.3)
Balance Without Adoption of Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue										7,787.1
Cost of services										6,220.6
Total costs and expenses										7,785.5
Operating income										1.6
Loss before income taxes										(221.8)
Benefit for income taxes										(29.6)
Net loss										(192.2)
Accounting Standards Update 2014-09 | Adoption Impact
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue										432.8
Cost of services										421.8
Total costs and expenses										421.8
Operating income										11.0
Loss before income taxes										11.0
Benefit for income taxes										4.6
Net loss										$ 6.4