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Revenue from Contracts with Customers - Summary of Consolidated Statement of Earnings Line Items, which Reflect Adoption of New Revenue Recognition Guidance (Detail) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Revenues before reimbursements | $ 1,741.2 | $ 1,559.9 | [1] | $ 5,169.1 | $ 4,627.5 | [1]
Total revenues | 1,778.5 | 1,593.7 | [1] | 5,276.6 | 4,729.6 | [1]
Compensation | 765.7 | 680.5 | [1] | 2,266.2 | 2,059.2 | [1]
Operating | 227.7 | 209.0 | [1] | 666.6 | 612.7 | [1]
Reimbursements | 37.3 | 33.8 | [1] | 107.5 | 102.1 | [1]
Interest | 36.7 | 31.4 | [1] | 101.9 | 92.9 | [1]
Depreciation | 33.4 | 30.5 | [1] | 94.8 | 90.2 | [1]
Amortization | 72.8 | 69.6 | [1] | 214.4 | 199.0 | [1]
Change in estimated acquisition earnout payables | 5.8 | 10.6 | [1] | 7.2 | 27.5 | [1]
Total expenses | 1,688.2 | 1,516.8 | [1] | 4,840.4 | 4,399.0 | [1]
Earnings before income taxes | 90.3 | 76.9 | [1] | 436.2 | 330.6 | [1]
Benefit for income taxes | (48.1) | (41.2) | [1] | (111.9) | (107.8) | [1]
Net earnings | 138.4 | 118.1 | [1] | 548.1 | 438.4 | [1]
Net earnings attributable to noncontrolling interests | 10.8 | 7.1 | [1] | 31.9 | 28.6 | [1]
Net earnings attributable to controlling interests | $ 127.6 | $ 111.0 | [1] | $ 516.2 | $ 409.8 | [1]
Basic net earnings per share | $ 0.70 | $ 0.61 | [1] | $ 2.83 | $ 2.28 | [1]
Diluted net earnings per share | 0.68 | 0.61 | [1] | 2.78 | 2.26 | [1]
Dividends declared per common share | $ 0.41 | $ 0.39 | [1] | $ 1.23 | $ 1.17 | [1]
Commissions [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | $ 707.6 | $ 637.9 | [1] | $ 2,235.0 | $ 2,016.8 | [1]
Broker Fees [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | 452.7 | 416.2 | [1] | 1,323.0 | 1,208.1 | [1]
Supplemental Revenue Member [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | 43.9 | 36.9 | [1] | 144.0 | 120.0 | [1]
Contingent Revenue [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | 25.7 | 21.8 | [1] | 82.4 | 78.1 | [1]
Investment income [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | 18.0 | 15.9 | [1] | 47.2 | 42.6 | [1]
Gains on Books of Business Sales [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | 0.6 | 0.6 | [1] | 9.6 | 3.1 | [1]
Clean Coal Activities [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | 492.4 | 430.6 | [1] | 1,327.0 | 1,158.8 | [1]
Cost of revenues from clean coal activities | 508.8 | 451.4 | [1] | 1,381.8 | 1,215.4 | [1]
Reimbursements [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues | $ 37.3 | 33.8 | [1] | $ 107.5 | 102.1 | [1]
As Previously Reported [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Revenues before reimbursements |  | 1,584.5 |  |  | 4,560.6 |
Total revenues |  | 1,584.5 |  |  | 4,560.6 |
Compensation |  | 680.1 |  |  | 2,013.4 |
Operating |  | 214.2 |  |  | 626.2 |
Interest |  | 31.4 |  |  | 92.9 |
Depreciation |  | 30.5 |  |  | 90.2 |
Amortization |  | 69.6 |  |  | 199.0 |
Change in estimated acquisition earnout payables |  | 10.6 |  |  | 27.5 |
Total expenses |  | 1,487.8 |  |  | 4,264.6 |
Earnings before income taxes |  | 96.7 |  |  | 296.0 |
Benefit for income taxes |  | (41.1) |  |  | (90.0) |
Net earnings |  | 137.8 |  |  | 386.0 |
Net earnings attributable to noncontrolling interests |  | 7.4 |  |  | 28.0 |
Net earnings attributable to controlling interests |  | $ 130.4 |  |  | $ 358.0 |
Basic net earnings per share |  | $ 0.72 |  |  | $ 1.99 |
Diluted net earnings per share |  | 0.71 |  |  | 1.97 |
Dividends declared per common share |  | $ 0.39 |  |  | $ 1.17 |
As Previously Reported [Member] | Commissions [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | $ 662.6 |  |  | $ 1,943.3 |
As Previously Reported [Member] | Broker Fees [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 422.4 |  |  | 1,203.8 |
As Previously Reported [Member] | Supplemental Revenue Member [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 39.9 |  |  | 115.9 |
As Previously Reported [Member] | Contingent Revenue [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 13.5 |  |  | 96.4 |
As Previously Reported [Member] | Investment income [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 14.9 |  |  | 39.3 |
As Previously Reported [Member] | Gains on Books of Business Sales [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 0.6 |  |  | 3.1 |
As Previously Reported [Member] | Clean Coal Activities [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 430.6 |  |  | 1,158.8 |
Cost of revenues from clean coal activities |  | 451.4 |  |  | 1,215.4 |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Revenues before reimbursements |  | (24.6) |  |  | 66.9 |
Total revenues |  | 9.2 |  |  | 169.0 |
Compensation |  | 0.4 |  |  | 45.8 |
Operating |  | (5.2) |  |  | (13.5) |
Reimbursements |  | 33.8 |  |  | 102.1 |
Total expenses |  | 29.0 |  |  | 134.4 |
Earnings before income taxes |  | (19.8) |  |  | 34.6 |
Benefit for income taxes |  | (0.1) |  |  | (17.8) |
Net earnings |  | (19.7) |  |  | 52.4 |
Net earnings attributable to noncontrolling interests |  | (0.3) |  |  | 0.6 |
Net earnings attributable to controlling interests |  | $ (19.4) |  |  | $ 51.8 |
Basic net earnings per share |  | $ (0.11) |  |  | $ 0.29 |
Diluted net earnings per share |  | $ (0.10) |  |  | $ 0.29 |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Commissions [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | $ (24.7) |  |  | $ 73.5 |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Broker Fees [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | (6.2) |  |  | 4.3 |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Supplemental Revenue Member [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | (3.0) |  |  | 4.1 |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Contingent Revenue [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 8.3 |  |  | (18.3) |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Investment income [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | 1.0 |  |  | 3.3 |
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Reimbursements [Member] |  |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |  |
Total revenues |  | $ 33.8 |  |  | $ 102.1 |
[1] | See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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