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Revenue from Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail)
$ in Millions
 3 Months Ended
Mar. 31, 2018
USD ($)
Deferred Revenue Arrangement [Line Items]  
Deferred revenue beginning balance $ 430.6
Incremental deferred revenue 182.5
Revenue recognized during the three-month period ended March 31, 2018 included in deferred revenue at December 31, 2017 (148.6)
Deferred revenue recognized from business acquisitions 0.7
Deferred revenue ending balance 465.2
Brokerage [Member]  
Deferred Revenue Arrangement [Line Items]  
Deferred revenue beginning balance 258.7
Incremental deferred revenue 144.6
Revenue recognized during the three-month period ended March 31, 2018 included in deferred revenue at December 31, 2017 (115.9)
Deferred revenue recognized from business acquisitions 0.7
Deferred revenue ending balance 288.1
Risk Management [Member]  
Deferred Revenue Arrangement [Line Items]  
Deferred revenue beginning balance 171.9
Incremental deferred revenue 37.9
Revenue recognized during the three-month period ended March 31, 2018 included in deferred revenue at December 31, 2017 (32.7)
Deferred revenue ending balance $ 177.1