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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |  |  |
Revenues before reimbursements | $ 1,625.0 | $ 1,454.3 | [1] | $ 3,427.9 | $ 3,067.6 | [1]
Total revenues | 1,660.4 | 1,489.5 | [1] | 3,498.1 | 3,135.9 | [1]
Commissions [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 688.0 | 618.2 | [1] | 1,527.4 | 1,378.9 | [1]
Broker Fees [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 422.2 | 386.9 | [1] | 870.3 | 791.9 | [1]
Supplemental Revenue Member [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 48.1 | 35.8 | [1] | 100.1 | 83.1 | [1]
Contingent Revenue [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 21.8 | 21.3 | [1] | 56.7 | 56.3 | [1]
Investment income [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 15.8 | 14.4 | [1] | 29.2 | 26.7 | [1]
Gains on Books of Business Sales [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 6.1 | 1.1 | [1] | 9.0 | 2.5 | [1]
Clean Coal Activities [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 422.4 | 376.4 | [1] | 834.6 | 728.2 | [1]
Product and Service, Other [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 0.6 | 0.2 | [1] | 0.6 |  |
Reimbursements [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 35.4 | 35.2 | [1] | 70.2 | 68.3 | [1]
Operating Segments [Member] | Brokerage [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Revenues before reimbursements | 1,000.1 | 896.1 |  | 2,195.7 | 1,981.0 |
Total revenues | 1,000.1 | 896.1 |  | 2,195.7 | 1,981.0 |
Operating Segments [Member] | Brokerage [Member] | Commissions [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 688.0 | 618.2 |  | 1,527.4 | 1,378.9 |
Operating Segments [Member] | Brokerage [Member] | Broker Fees [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 220.4 | 205.5 |  | 473.6 | 433.8 |
Operating Segments [Member] | Brokerage [Member] | Supplemental Revenue Member [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 48.1 | 35.8 |  | 100.1 | 83.1 |
Operating Segments [Member] | Brokerage [Member] | Contingent Revenue [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 21.8 | 21.3 |  | 56.7 | 56.3 |
Operating Segments [Member] | Brokerage [Member] | Investment income [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 15.7 | 14.2 |  | 28.9 | 26.4 |
Operating Segments [Member] | Brokerage [Member] | Gains on Books of Business Sales [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 6.1 | 1.1 |  | 9.0 | 2.5 |
Operating Segments [Member] | Risk Management [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Revenues before reimbursements | 201.9 | 181.6 |  | 397.0 | 358.4 |
Total revenues | 237.3 | 216.8 |  | 467.2 | 426.7 |
Operating Segments [Member] | Risk Management [Member] | Broker Fees [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 201.8 | 181.4 |  | 396.7 | 358.1 |
Operating Segments [Member] | Risk Management [Member] | Investment income [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 0.1 | 0.2 |  | 0.3 | 0.3 |
Operating Segments [Member] | Risk Management [Member] | Reimbursements [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 35.4 | 35.2 |  | 70.2 | 68.3 |
Operating Segments [Member] | Corporate [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Revenues before reimbursements | 423.0 | 376.6 |  | 835.2 | 728.2 |
Total revenues | 423.0 | 376.6 |  | 835.2 | 728.2 |
Operating Segments [Member] | Corporate [Member] | Clean Coal Activities [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | 422.4 | 376.4 |  | 834.6 | $ 728.2 |
Operating Segments [Member] | Corporate [Member] | Product and Service, Other [Member] |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |
Total revenues | $ 0.6 | $ 0.2 |  | $ 0.6 |  |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

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- Definition Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045 + Details
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