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v3.21.2

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | $ 1,896.8 | $ 1,551.6 | $ 4,029.4 | $ 3,380.8
Reimbursements | 36.7 | 32.4 | 69.1 | 70.1
Total revenues | 1,933.5 | 1,584.0 | 4,098.5 | 3,450.9
Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,390.2 | 1,201.1 | 3,000.4 | 2,636.7
Total revenues | 1,390.2 | 1,201.1 | 3,000.4 | 2,636.7
Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 245.0 | 190.8 | 465.3 | 402.6
Reimbursements | 36.7 | 32.4 | 69.1 | 70.1
Total revenues | 281.7 | 223.2 | 534.4 | 472.7
Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 261.6 | 159.7 | 563.7 | 341.5
Total revenues | 261.6 | 159.7 | 563.7 | 341.5
Commissions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 977.1 | 827.5 | 2,102.5 | 1,844.7
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 977.1 | 827.5 | 2,102.5 | 1,844.7
Broker Fees [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 540.9 | 459.7 | 1,094.7 | 967.0
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 296.0 | 269.1 | 629.6 | 564.9
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 244.9 | 190.6 | 465.1 | 402.1
Supplemental Revenue Member [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 55.2 | 50.3 | 122.0 | 109.3
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 55.2 | 50.3 | 122.0 | 109.3
Contingent Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 43.3 | 37.4 | 106.6 | 82.5
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 43.3 | 37.4 | 106.6 | 82.5
Investment Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 18.1 | 16.0 | 35.2 | 34.6
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 18.1 | 15.8 | 35.1 | 34.1
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements |  | 0.2 | 0.1 | 0.5
Net Gains on Divestitures [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.6 | 1.0 | 4.7 | 1.2
Net Gains on Divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.5 | 1.0 | 4.6 | 1.2
Net Gains on Divestitures [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.1 |  | 0.1 |
Clean Coal Activities [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 261.0 | 159.5 | 561.6 | 341.3
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 261.0 | 159.5 | 561.6 | 341.3
Other Net Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 0.6 | 0.2 | 2.1 | 0.2
Other Net Revenue [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | $ 0.6 | $ 0.2 | $ 2.1 | $ 0.2

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