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v3.22.2.2

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | $ 2,012.0 | $ 2,105.6 | $ 6,424.9 | $ 6,135.0
Reimbursements | 32.0 | 32.6 | 97.4 | 101.7
Total revenues | 2,044.0 | 2,138.2 | 6,522.3 | 6,236.7
Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,736.2 | 1,499.7 | 5,599.5 | 4,500.1
Total revenues | 1,736.2 | 1,499.7 | 5,599.5 | 4,500.1
Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 275.5 | 248.0 | 802.0 | 713.3
Reimbursements | 32.0 | 32.6 | 97.4 | 101.7
Total revenues | 307.5 | 280.6 | 899.4 | 815.0
Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.3 | 357.9 | 23.4 | 921.6
Total revenues | 0.3 | 357.9 | 23.4 | 921.6
Commissions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,186.9 | 1,016.2 | 4,034.6 | 3,118.7
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,186.9 | 1,016.2 | 4,034.6 | 3,118.7
Broker Fees [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 671.3 | 601.7 | 1,913.1 | 1,696.4
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 396.0 | 353.8 | 1,111.5 | 983.4
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 275.3 | 247.9 | 801.6 | 713.0
Supplemental Revenue Member [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 64.7 | 61.0 | 204.7 | 183.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 64.7 | 61.0 | 204.7 | 183.0
Contingent Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 52.4 | 43.7 | 167.1 | 150.3
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 52.4 | 43.7 | 167.1 | 150.3
Investment Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 37.7 | 20.8 | 79.1 | 56.0
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 37.5 | 20.7 | 78.7 | 55.8
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.2 | 0.1 | 0.4 | 0.2
Net losses on divestitures [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | (1.3) | 4.3 | 2.9 | 9.0
Net losses on divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | (1.3) | 4.3 | 2.9 | 8.9
Net losses 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 |  | 357.6 | 23.0 | 919.2
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements |  | 357.6 | 23.0 | 919.2
Other Net Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.3 | 0.3 | 0.4 | 2.4
Other Net Revenue [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | $ 0.3 | $ 0.3 | $ 0.4 | $ 2.4

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