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Revenue from Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 430.6
Incremental deferred revenue	340.9
Revenue recognized during the nine-month period ended September 30, 2018 included in deferred revenue at December 31, 2017	(312.6)
Deferred revenue recognized from business acquisitions	13.8
Deferred revenue ending balance	472.7
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	258.7
Incremental deferred revenue	231.7
Revenue recognized during the nine-month period ended September 30, 2018 included in deferred revenue at December 31, 2017	(207.2)
Deferred revenue recognized from business acquisitions	13.8
Deferred revenue ending balance	297.0
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	171.9
Incremental deferred revenue	109.2
Revenue recognized during the nine-month period ended September 30, 2018 included in deferred revenue at December 31, 2017	(105.4)
Deferred revenue ending balance	$ 175.7