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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 1,837.7	$ 1,646.4	[1]
Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	1,195.6	1,084.9
Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	229.9	209.9
Operating Segments [Member] | Corporate [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	412.2	351.6
Unites States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	1,457.7	1,327.0
Unites States [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	852.8	793.0
Unites States [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	192.7	182.4
Unites States [Member] | Operating Segments [Member] | Corporate [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	412.2	351.6
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	200.0	167.7
United Kingdom [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	191.0	161.3
United Kingdom [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	9.0	6.4
Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	67.3	59.0
Australia [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	43.6	40.3
Australia [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	23.7	18.7
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	43.8	32.9
Canada [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	42.8	32.1
Canada [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	1.0	0.8
New Zealand [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	33.6	28.7
New Zealand [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	30.1	27.1
New Zealand [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	3.5	1.6
Other Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	35.3	31.1
Other Foreign [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 35.3	$ 31.1
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

X

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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