Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2022 USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 579.6
Incremental deferred revenue	376.8
Revenue recognized during the six-month period ended June 30, 2022 included in deferred revenue at December 31, 2021	(324.4)
Net change in collected billings/deposits received from customers	(1.8)
Impact of change in foreign exchange rates	(13.5)
Deferred revenue recognized from business acquisitions	5.5
Deferred revenue ending balance	622.2
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	395.9
Incremental deferred revenue	324.5
Revenue recognized during the six-month period ended June 30, 2022 included in deferred revenue at December 31, 2021	(272.1)
Net change in collected billings/deposits received from customers	5.0
Impact of change in foreign exchange rates	(13.5)
Deferred revenue recognized from business acquisitions	5.5
Deferred revenue ending balance	445.3
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	183.7
Incremental deferred revenue	52.3
Revenue recognized during the six-month period ended June 30, 2022 included in deferred revenue at December 31, 2021	(52.3)
Net change in collected billings/deposits received from customers	(6.8)
Deferred revenue ending balance	$ 176.9