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v3.19.3

Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | $ 1,790.5 | $ 1,741.2 | $ 5,372.8 | $ 5,169.1
Total revenues | 1,825.2 | 1,778.5 | 5,473.6 | 5,276.6
Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 1,195.9 | 1,049.4 | 3,709.0 | 3,245.1
Total revenues | 1,195.9 | 1,049.4 | 3,709.0 | 3,245.1
Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 211.6 | 199.1 | 624.0 | 596.1
Total revenues | 246.3 | 236.4 | 724.8 | 703.6
Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues before reimbursements | 383.0 | 492.7 | 1,039.8 | 1,327.9
Total revenues | 383.0 | 492.7 | 1,039.8 | 1,327.9
Commissions [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 809.9 | 707.6 | 2,528.0 | 2,235.0
Commissions [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 809.9 | 707.6 | 2,528.0 | 2,235.0
Broker Fees [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 490.1 | 452.7 | 1,419.5 | 1,323.0
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 278.9 | 253.7 | 796.8 | 727.3
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 211.2 | 199.0 | 622.7 | 595.7
Supplemental Revenue Member [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 49.8 | 43.9 | 153.4 | 144.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 49.8 | 43.9 | 153.4 | 144.0
Contingent Revenue [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 30.4 | 25.7 | 107.9 | 82.4
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 30.4 | 25.7 | 107.9 | 82.4
Investment income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 24.0 | 18.0 | 61.9 | 47.2
Investment income [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 23.6 | 17.9 | 60.6 | 46.8
Investment income [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 0.4 | 0.1 | 1.3 | 0.4
Net gains on divestitures [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 3.3 | 0.6 | 62.3 | 9.6
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 3.3 | 0.6 | 62.3 | 9.6
Clean Coal Activities [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 386.0 | 492.4 | 1,042.7 | 1,327.0
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 386.0 | 492.4 | 1,042.7 | 1,327.0
Other net gains [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | (3.0) | 0.3 | (2.9) | 0.9
Other net gains [Member] | Corporate [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | (3.0) | 0.3 | (2.9) | 0.9
Reimbursements [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 34.7 | 37.3 | 100.8 | 107.5
Reimbursements [Member] | Risk Management [Member] | Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | $ 34.7 | $ 37.3 | $ 100.8 | $ 107.5

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