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v3.22.1

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | $ 2,404.5 | $ 2,132.6
Reimbursements | 30.8 | 32.4
Total revenues | 2,435.3 | 2,165.0
Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 2,122.6 | 1,610.2
Total revenues | 2,122.6 | 1,610.2
Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 259.1 | 220.3
Reimbursements | 30.8 | 32.4
Total revenues | 289.9 | 252.7
Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 22.8 | 302.1
Total revenues | 22.8 | 302.1
Commissions [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 1,565.3 | 1,125.4
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 1,565.3 | 1,125.4
Broker Fees [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 650.9 | 553.8
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 391.9 | 333.6
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 259.0 | 220.2
Supplemental Revenue Member [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 74.3 | 66.8
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 74.3 | 66.8
Contingent Revenue [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 71.6 | 63.3
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 71.6 | 63.3
Investment Income [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 18.2 | 17.1
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 18.1 | 17.0
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.1 | 0.1
Net Gains on Divestitures [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 1.4 | 4.1
Net Gains on Divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 1.4 | 4.1
Clean Coal Activities [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 22.7 | 300.6
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 22.7 | 300.6
Other Net Revenue (Losses) [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.1 | 1.5
Other Net Revenue (Losses) [Member] | Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | $ 0.1 | $ 1.5

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