Goodwill and Intangible Assets - Summary of Change in Goodwill (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 3,361.5
Current period acquisitions	42.9
Purchase accounting reclassification	(0.1)
Foreign currency translation	(3.6)
Goodwill, ending balance	3,400.7
Accumulated impairment loss		$ 3.2
Operating Segments | Insurance
Goodwill [Roll Forward]
Goodwill, beginning balance	833.8
Current period acquisitions	0.0
Purchase accounting reclassification	0.0
Foreign currency translation	(2.2)
Goodwill, ending balance	831.6
Operating Segments | Energy and Specialized Markets
Goodwill [Roll Forward]
Goodwill, beginning balance	2,054.7
Current period acquisitions	42.9
Purchase accounting reclassification	0.0
Foreign currency translation	(1.6)
Goodwill, ending balance	2,096.0
Operating Segments | Financial Services
Goodwill [Roll Forward]
Goodwill, beginning balance	473.0
Current period acquisitions	0.0
Purchase accounting reclassification	(0.1)
Foreign currency translation	0.2
Goodwill, ending balance	$ 473.1