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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, 2020 | Dec. 31, 2019
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | $ 503.8 | $ 457.7
Incremental deferred revenue | 374.0 | 364.3
Revenue recognized during the year included in deferred revenue of prior year | (382.3) | (349.5)
Net change in collected billings/deposits received from customers | 25.6 | (1.1)
Impact of changes in foreign exchange rates | 10.3 | 1.8
Deferred revenue recognized from business acquisitions | 9.9 | 30.6
Deferred revenue ending balance | 541.3 | 503.8
Brokerage [Member] |  |
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | 337.2 | 284.7
Incremental deferred revenue | 282.7 | 254.7
Revenue recognized during the year included in deferred revenue of prior year | (283.1) | (227.5)
Net change in collected billings/deposits received from customers | (0.7) | (6.7)
Impact of changes in foreign exchange rates | 8.7 | 1.4
Deferred revenue recognized from business acquisitions | 9.9 | 30.6
Deferred revenue ending balance | 354.7 | 337.2
Risk Management [Member] |  |
Deferred Revenue Arrangement [Line Items] |  |
Deferred revenue beginning balance | 166.6 | 173.0
Incremental deferred revenue | 91.3 | 109.6
Revenue recognized during the year included in deferred revenue of prior year | (99.2) | (122.0)
Net change in collected billings/deposits received from customers | 26.3 | 5.6
Impact of changes in foreign exchange rates | 1.6 | 0.4
Deferred revenue ending balance | $ 186.6 | $ 166.6

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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=121556615&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=121604090&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=121556615&loc=SL49130532-203044 + Details
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