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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | [1]
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | $ 1,837.7 | $ 1,646.4 | [1] |
Earnings before income taxes | 242.3 | 199.6 | [1] |
Identifiable assets at March 31, 2018 and 2017 | 16,127.5 |  |  | $ 14,909.7
Operating Segments [Member] | Brokerage [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 1,195.6 | 1,084.9 |  |
Earnings before income taxes | 318.9 | 276.5 |  |
Identifiable assets at March 31, 2018 and 2017 | 13,784.2 | 12,832.8 |  |
Operating Segments [Member] | Risk Management [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 229.9 | 209.9 |  |
Earnings before income taxes | 21.6 | 22.1 |  |
Identifiable assets at March 31, 2018 and 2017 | 735.9 | 697.7 |  |
Operating Segments [Member] | Corporate [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 412.2 | 351.6 |  |
Earnings before income taxes | (98.2) | (99.0) |  |
Identifiable assets at March 31, 2018 and 2017 | $ 1,607.4 | $ 1,659.4 |  |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

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