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v3.23.1

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | $ 2,672.9 | $ 2,404.5
Reimbursements | 33.2 | 30.8
Total revenues | 2,706.1 | 2,435.3
Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 2,375.2 | 2,122.6
Total revenues | 2,375.2 | 2,122.6
Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 297.6 | 259.1
Reimbursements | 33.2 | 30.8
Total revenues | 330.8 | 289.9
Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.1 | 22.8
Total revenues | 0.1 | 22.8
Commissions [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 1,747.4 | 1,565.3
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 1,747.4 | 1,565.3
Broker Fees [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 705.7 | 650.9
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 412.7 | 391.9
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 293.0 | 259.0
Supplemental Revenue Member [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 81.6 | 74.3
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 81.6 | 74.3
Contingent Revenue [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 71.8 | 71.6
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 71.8 | 71.6
Investment Income [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 66.0 | 18.2
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 61.5 | 18.1
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 4.5 | 0.1
Net gains on divestitures [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.3 | 1.4
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.2 | 1.4
Net gains on divestitures [Member] | Risk Management [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.1 |
Clean Coal Activities [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements |  | 22.7
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements |  | 22.7
Other Net Revenue [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | 0.1 | 0.1
Other Net Revenue [Member] | Corporate [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues before reimbursements | $ 0.1 | $ 0.1

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