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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |
Total revenues | $ 1,412.7 | $ 1,300.4 |
Earnings before income taxes | 52.3 | 55.7 |
Identifiable assets at March 31, 2017 and 2016 | 11,780.6 |  | $ 11,489.6
Operating Segments [Member] | Brokerage [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total revenues | 878.5 | 825.5 |
Earnings before income taxes | 117.7 | 100.0 |
Identifiable assets at March 31, 2017 and 2016 | 9,422.6 | 8,681.9 |
Operating Segments [Member] | Risk Management [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total revenues | 182.6 | 179.3 |
Earnings before income taxes | 22.9 | 24.0 |
Identifiable assets at March 31, 2017 and 2016 | 691.1 | 690.4 |
Operating Segments [Member] | Corporate [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total revenues | 351.6 | 295.6 |
Earnings before income taxes | (88.3) | (68.3) |
Identifiable assets at March 31, 2017 and 2016 | $ 1,666.9 | $ 1,340.3 |

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