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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Segment Reporting Information [Line Items]    
Commissions $ 839.4 $ 760.7 [1]
Fees 448.1 405.0 [1]
Supplemental revenues 52.0 47.3 [1]
Contingent revenues 34.9 35.0 [1]
Investment income 13.4 12.3 [1]
Gains on books of business sales 2.9 1.4 [1]
Revenues from clean coal activities 412.2 351.8 [1]
Other net losses [1]   (0.2)
Revenues before reimbursements 1,802.9 1,613.3 [1]
Reimbursements 34.8 33.1 [1]
Total revenues 1,837.7 1,646.4 [1]
Operating Segments [Member] | Brokerage [Member]    
Segment Reporting Information [Line Items]    
Commissions 839.4 760.7
Fees 253.2 228.3
Supplemental revenues 52.0 47.3
Contingent revenues 34.9 35.0
Investment income 13.2 12.2
Gains on books of business sales 2.9 1.4
Revenues before reimbursements 1,195.6 1,084.9
Total revenues 1,195.6 1,084.9
Operating Segments [Member] | Risk Management [Member]    
Segment Reporting Information [Line Items]    
Fees 194.9 176.7
Investment income 0.2 0.1
Revenues before reimbursements 195.1 176.8
Reimbursements 34.8 33.1
Total revenues 229.9 209.9
Operating Segments [Member] | Corporate [Member]    
Segment Reporting Information [Line Items]    
Revenues from clean coal activities 412.2 351.8
Other net losses   (0.2)
Revenues before reimbursements 412.2 351.6
Total revenues $ 412.2 $ 351.6
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.