Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 503.8	$ 457.7
Incremental deferred revenue	374.0	364.3
Revenue recognized during the year included in deferred revenue of prior year	(382.3)	(349.5)
Net change in collected billings/deposits received from customers	25.6	(1.1)
Impact of changes in foreign exchange rates	10.3	1.8
Deferred revenue recognized from business acquisitions	9.9	30.6
Deferred revenue ending balance	541.3	503.8
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	337.2	284.7
Incremental deferred revenue	282.7	254.7
Revenue recognized during the year included in deferred revenue of prior year	(283.1)	(227.5)
Net change in collected billings/deposits received from customers	(0.7)	(6.7)
Impact of changes in foreign exchange rates	8.7	1.4
Deferred revenue recognized from business acquisitions	9.9	30.6
Deferred revenue ending balance	354.7	337.2
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	166.6	173.0
Incremental deferred revenue	91.3	109.6
Revenue recognized during the year included in deferred revenue of prior year	(99.2)	(122.0)
Net change in collected billings/deposits received from customers	26.3	5.6
Impact of changes in foreign exchange rates	1.6	0.4
Deferred revenue ending balance	$ 186.6	$ 166.6