Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 579.6
Incremental deferred revenue	281.4
Revenue recognized during the three-month period ended March 31, 2022 included in deferred revenue at December 31, 2021	(237.2)
Net change in collected billings/deposits received from customers	14.4
Impact of change in foreign exchange rates	1.0
Deferred revenue recognized from business acquisitions	3.1
Deferred revenue ending balance	642.3
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	395.9
Incremental deferred revenue	253.6
Revenue recognized during the three-month period ended March 31, 2022 included in deferred revenue at December 31, 2021	(210.5)
Net change in collected billings/deposits received from customers	14.7
Impact of change in foreign exchange rates	0.5
Deferred revenue recognized from business acquisitions	3.1
Deferred revenue ending balance	457.3
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	183.7
Incremental deferred revenue	27.8
Revenue recognized during the three-month period ended March 31, 2022 included in deferred revenue at December 31, 2021	(26.7)
Net change in collected billings/deposits received from customers	(0.3)
Impact of change in foreign exchange rates	0.5
Deferred revenue ending balance	$ 185.0