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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | $ 1,825.2 | $ 1,778.5 | $ 5,473.6 | $ 5,276.6 |
Earnings before income taxes | 114.8 | 90.3 | 542.1 | 436.2 |
Identifiable assets at September 30, 2019 and 2018 | 19,173.8 | 16,114.7 | 19,173.8 | 16,114.7 | $ 16,334.0
Operating Segments [Member] | Brokerage [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | 1,195.9 | 1,049.4 | 3,709.0 | 3,245.1 |
Earnings before income taxes | 199.9 | 163.2 | 794.6 | 652.7 |
Identifiable assets at September 30, 2019 and 2018 | 16,261.6 | 13,700.7 | 16,261.6 | 13,700.7 |
Operating Segments [Member] | Risk Management [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | 246.3 | 236.4 | 724.8 | 703.6 |
Earnings before income taxes | 21.7 | 22.3 | 64.7 | 67.9 |
Identifiable assets at September 30, 2019 and 2018 | 920.8 | 748.3 | 920.8 | 748.3 |
Operating Segments [Member] | Corporate [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | 383.0 | 492.7 | 1,039.8 | 1,327.9 |
Earnings before income taxes | (106.8) | (95.2) | (317.2) | (284.4) |
Identifiable assets at September 30, 2019 and 2018 | $ 1,991.4 | $ 1,665.7 | $ 1,991.4 | $ 1,665.7 |

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