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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Segment Reporting Information [Line Items] |  |  |  |
Commissions |  | $ 839.4 | $ 760.7 | [1]
Fees |  | 448.1 | 405.0 | [1]
Supplemental revenues |  | 52.0 | 47.3 | [1]
Contingent revenues |  | 34.9 | 35.0 | [1]
Investment income |  | 13.4 | 12.3 | [1]
Gains on books of business sales |  | 2.9 | 1.4 | [1]
Revenues from clean coal activities |  | 412.2 | 351.8 | [1]
Other net losses | [1] |  | (0.2) |
Revenues before reimbursements |  | 1,802.9 | 1,613.3 | [1]
Reimbursements |  | 34.8 | 33.1 | [1]
Total revenues |  | 1,837.7 | 1,646.4 | [1]
Operating Segments [Member] | Brokerage [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Commissions |  | 839.4 | 760.7 |
Fees |  | 253.2 | 228.3 |
Supplemental revenues |  | 52.0 | 47.3 |
Contingent revenues |  | 34.9 | 35.0 |
Investment income |  | 13.2 | 12.2 |
Gains on books of business sales |  | 2.9 | 1.4 |
Revenues before reimbursements |  | 1,195.6 | 1,084.9 |
Total revenues |  | 1,195.6 | 1,084.9 |
Operating Segments [Member] | Risk Management [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Fees |  | 194.9 | 176.7 |
Investment income |  | 0.2 | 0.1 |
Revenues before reimbursements |  | 195.1 | 176.8 |
Reimbursements |  | 34.8 | 33.1 |
Total revenues |  | 229.9 | 209.9 |
Operating Segments [Member] | Corporate [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues from clean coal activities |  | 412.2 | 351.8 |
Other net losses |  |  | (0.2) |
Revenues before reimbursements |  | 412.2 | 351.6 |
Total revenues |  | $ 412.2 | $ 351.6 |
[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 of commission revenue from buying and selling securities on behalf of customers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 20 -Section 05 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=68070981&loc=d3e40544-110947 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(a),(b)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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- Definition The difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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