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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | $ 1,584.5 | $ 1,482.3 | $ 4,560.6 | $ 4,209.8 |
Earnings before income taxes | 96.7 | 94.9 | 296.0 | 288.1 |
Identifiable assets at September 30, 2017 and 2016 | 12,810.4 |  | 12,810.4 |  | $ 11,489.6
Operating Segments [Member] | Brokerage [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | 953.7 | 877.6 | 2,830.3 | 2,642.2 |
Earnings before income taxes | 161.6 | 149.1 | 491.8 | 427.3 |
Identifiable assets at September 30, 2017 and 2016 | 10,275.9 | 8,972.7 | 10,275.9 | 8,972.7 |
Operating Segments [Member] | Risk Management [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | 200.2 | 176.7 | 571.5 | 532.5 |
Earnings before income taxes | 27.8 | 21.5 | 73.6 | 67.9 |
Identifiable assets at September 30, 2017 and 2016 | 733.0 | 679.6 | 733.0 | 679.6 |
Operating Segments [Member] | Corporate [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Total revenues | 430.6 | 428.0 | 1,158.8 | 1,035.1 |
Earnings before income taxes | (92.7) | (75.7) | (269.4) | (207.1) |
Identifiable assets at September 30, 2017 and 2016 | $ 1,801.5 | $ 1,548.0 | $ 1,801.5 | $ 1,548.0 |

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