Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations 1 (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 541.3
Brokerage [Member]
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	354.7
Risk Management [Member]
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 186.6