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v3.20.2

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | $ 1,584.0 | $ 1,657.8 | $ 3,450.9 | $ 3,648.4
Revenues before reimbursements | 1,551.6 | 1,624.8 | 3,380.8 | 3,582.3
Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 1,201.1 | 1,131.2 | 2,636.7 | 2,513.1
Revenues before reimbursements | 1,201.1 | 1,131.2 | 2,636.7 | 2,513.1
Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 223.2 | 242.1 | 472.7 | 478.5
Revenues before reimbursements | 190.8 | 209.1 | 402.6 | 412.4
Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 159.7 | 284.5 | 341.5 | 656.8
Revenues before reimbursements | 159.7 | 284.5 | 341.5 | 656.8
Commissions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 827.5 | 777.7 | 1,844.7 | 1,718.1
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 827.5 | 777.7 | 1,844.7 | 1,718.1
Broker Fees [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 459.7 | 464.7 | 967.0 | 929.4
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 269.1 | 256.1 | 564.9 | 517.9
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 190.6 | 208.6 | 402.1 | 411.5
Supplemental Revenue Member [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 50.3 | 46.9 | 109.3 | 103.6
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 50.3 | 46.9 | 109.3 | 103.6
Contingent Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 37.4 | 29.5 | 82.5 | 77.5
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 37.4 | 29.5 | 82.5 | 77.5
Investment income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 16.0 | 19.6 | 34.6 | 37.9
Investment income [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 15.8 | 19.1 | 34.1 | 37.0
Investment income [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 0.2 | 0.5 | 0.5 | 0.9
Net gains on divestitures [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 1.0 | 1.9 | 1.2 | 59.0
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 1.0 | 1.9 | 1.2 | 59.0
Clean Coal Activities [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 159.5 | 284.4 | 341.3 | 656.7
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 159.5 | 284.4 | 341.3 | 656.7
Other Net Losses [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 0.2 | 0.1 | 0.2 | 0.1
Other Net Losses [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 0.2 | 0.1 | 0.2 | 0.1
Reimbursements [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 32.4 | 33.0 | 70.1 | 66.1
Reimbursements [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | $ 32.4 | $ 33.0 | $ 70.1 | $ 66.1

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