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Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Deferred Revenue Arrangement [Line Items]    
Deferred revenue beginning balance $ 457.7 $ 430.6
Incremental deferred revenue 413.1 382.1
Revenue recognized during the year included in deferred revenue of prior year (397.6) (373.5)
Deferred revenue recognized from business acquisitions 30.6 18.5
Deferred revenue ending balance 503.8 457.7
Brokerage [Member]    
Deferred Revenue Arrangement [Line Items]    
Deferred revenue beginning balance 284.7 258.7
Incremental deferred revenue 276.2 244.0
Revenue recognized during the year included in deferred revenue of prior year (254.3) (236.5)
Deferred revenue recognized from business acquisitions 30.6 18.5
Deferred revenue ending balance 337.2 284.7
Risk Management [Member]    
Deferred Revenue Arrangement [Line Items]    
Deferred revenue beginning balance 173.0 171.9
Incremental deferred revenue 136.9 138.1
Revenue recognized during the year included in deferred revenue of prior year (143.3) (137.0)
Deferred revenue ending balance $ 166.6 $ 173.0