Revenue from Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 430.6	$ 404.6
Incremental deferred revenue	382.1	382.9
Revenue recognized during the year ended December 31, 2017 included in deferred revenue at December 31, 2016	(373.5)	(362.8)
Deferred revenue recognized from business acquisitions	18.5	5.9
Deferred revenue ending balance	457.7	430.6
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	258.7	233.8
Incremental deferred revenue	244.0	247.2
Revenue recognized during the year ended December 31, 2017 included in deferred revenue at December 31, 2016	(236.5)	(227.8)
Deferred revenue recognized from business acquisitions	18.5	5.5
Deferred revenue ending balance	284.7	258.7
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	171.9	170.8
Incremental deferred revenue	138.1	135.7
Revenue recognized during the year ended December 31, 2017 included in deferred revenue at December 31, 2016	(137.0)	(135.0)
Deferred revenue recognized from business acquisitions		0.4
Deferred revenue ending balance	$ 173.0	$ 171.9