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v3.23.2

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | $ 2,406.9 | $ 2,008.4 | $ 5,079.8 | $ 4,412.9
Reimbursements | 35.0 | 34.6 | 68.2 | 65.4
Total revenues | 2,441.9 | 2,043.0 | 5,148.0 | 4,478.3
Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 2,088.4 | 1,740.7 | 4,463.6 | 3,863.3
Total revenues | 2,088.4 | 1,740.7 | 4,463.6 | 3,863.3
Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 318.6 | 267.4 | 616.2 | 526.5
Reimbursements | 35.0 | 34.6 | 68.2 | 65.4
Total revenues | 353.6 | 302.0 | 684.4 | 591.9
Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | (0.1) | 0.3 |  | 23.1
Total revenues | (0.1) | 0.3 |  | 23.1
Commissions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,410.4 | 1,282.4 | 3,157.8 | 2,847.7
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,410.4 | 1,282.4 | 3,157.8 | 2,847.7
Broker Fees [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 790.5 | 590.9 | 1,496.2 | 1,241.8
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 478.5 | 323.6 | 891.2 | 715.5
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 312.0 | 267.3 | 605.0 | 526.3
Supplemental Revenue Member [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 71.2 | 65.7 | 152.8 | 140.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 71.2 | 65.7 | 152.8 | 140.0
Contingent Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 54.2 | 43.1 | 126.0 | 114.7
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 54.2 | 43.1 | 126.0 | 114.7
Investment Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 75.6 | 23.2 | 141.6 | 41.4
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 69.1 | 23.1 | 130.6 | 41.2
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 6.5 | 0.1 | 11.0 | 0.2
Net gains on divestitures [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 5.1 | 2.8 | 5.4 | 4.2
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 5.0 | 2.8 | 5.2 | 4.2
Net gains on divestitures [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.1 |  | $ 0.2 |
Clean Coal Activities [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements |  | 0.3 |  | 23.0
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements |  | $ 0.3 |  | 23.0
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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