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Revenue from Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 430.6
Incremental deferred revenue	277.5
Revenue recognized during the six-month period ended June 30, 2018 included in deferred revenue at December 31, 2017	(249.2)
Deferred revenue recognized from business acquisitions	12.9
Deferred revenue ending balance	471.8
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	258.7
Incremental deferred revenue	204.1
Revenue recognized during the six-month period ended June 30, 2018 included in deferred revenue at December 31, 2017	(181.0)
Deferred revenue recognized from business acquisitions	12.9
Deferred revenue ending balance	294.7
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	171.9
Incremental deferred revenue	73.4
Revenue recognized during the six-month period ended June 30, 2018 included in deferred revenue at December 31, 2017	(68.2)
Deferred revenue ending balance	$ 177.1

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- Definition

Deferred revenue, additions through business acquisitions.

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- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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- Definition

Amount of deferred revenue recognized for transactions arising during the current reporting period. Deferred revenue is a liability as of the balance sheet date related to a revenue producing activity for which revenue has not yet been recognized. Generally, an Entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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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.

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- Definition

Amount of revenue recognized that was previously reported as deferred or unearned revenue.

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