Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2024 USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 706.2
Incremental deferred revenue	463.0
Revenue recognized during the six-month period ended June 30, 2024 included in deferred revenue at December 31, 2023	(385.6)
Net change in collected billings/deposits received from customers	(23.7)
Impact of change in foreign exchange rates	(2.3)
Deferred revenue recognized from business acquisitions	5.3
Deferred revenue ending balance	762.9
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	533.6
Incremental deferred revenue	417.5
Revenue recognized during the six-month period ended June 30, 2024 included in deferred revenue at December 31, 2023	(343.5)
Net change in collected billings/deposits received from customers	(22.8)
Impact of change in foreign exchange rates	(2.2)
Deferred revenue recognized from business acquisitions	5.3
Deferred revenue ending balance	587.9
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	172.6
Incremental deferred revenue	45.5
Revenue recognized during the six-month period ended June 30, 2024 included in deferred revenue at December 31, 2023	(42.1)
Net change in collected billings/deposits received from customers	(0.9)
Impact of change in foreign exchange rates	(0.1)
Deferred revenue ending balance	$ 175.0