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Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | $ 457.7 | $ 430.6
Incremental deferred revenue | 413.1 | 382.1
Revenue recognized during the year included in deferred revenue of prior year | (397.6) | (373.5)
Deferred revenue recognized from business acquisitions | 30.6 | 18.5
Deferred revenue ending balance | 503.8 | 457.7
Brokerage [Member] |  |
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | 284.7 | 258.7
Incremental deferred revenue | 276.2 | 244.0
Revenue recognized during the year included in deferred revenue of prior year | (254.3) | (236.5)
Deferred revenue recognized from business acquisitions | 30.6 | 18.5
Deferred revenue ending balance | 337.2 | 284.7
Risk Management [Member] |  |
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | 173.0 | 171.9
Incremental deferred revenue | 136.9 | 138.1
Revenue recognized during the year included in deferred revenue of prior year | (143.3) | (137.0)
Deferred revenue ending balance | $ 166.6 | $ 173.0

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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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130532-203044 + Details
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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://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118956577&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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045 + Details
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