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Revenue from Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | $ 430.6 | $ 404.6
Incremental deferred revenue | 382.1 | 382.9
Revenue recognized during the year ended December 31, 2017 included in deferred revenue at December 31, 2016 | (373.5) | (362.8)
Deferred revenue recognized from business acquisitions | 18.5 | 5.9
Deferred revenue ending balance | 457.7 | 430.6
Brokerage [Member] |  |
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | 258.7 | 233.8
Incremental deferred revenue | 244.0 | 247.2
Revenue recognized during the year ended December 31, 2017 included in deferred revenue at December 31, 2016 | (236.5) | (227.8)
Deferred revenue recognized from business acquisitions | 18.5 | 5.5
Deferred revenue ending balance | 284.7 | 258.7
Risk Management [Member] |  |
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | 171.9 | 170.8
Incremental deferred revenue | 138.1 | 135.7
Revenue recognized during the year ended December 31, 2017 included in deferred revenue at December 31, 2016 | (137.0) | (135.0)
Deferred revenue recognized from business acquisitions |  | 0.4
Deferred revenue ending balance | $ 173.0 | $ 171.9

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- Definition Contract with customer, liability, additions. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045 + Details
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Balance Type: | credit
Period Type: | instant

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- Definition Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration from customer has been received or is due, from business combination. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130551-203045 + Details
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- Definition Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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- Details
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