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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 1,990.6	$ 1,837.7
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,577.3	1,457.7
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	213.8	200.0
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	68.5	67.3
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	58.5	43.8
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	34.3	33.6
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	38.2	35.3
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,381.9	1,195.6
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,006.5	852.8
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	203.6	191.0
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	46.0	43.6
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	57.3	42.8
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	30.3	30.1
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	38.2	35.3
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	236.4	229.9
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	198.5	192.7
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10.2	9.0
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	22.5	23.7
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.2	1.0
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	4.0	3.5
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	372.3	412.2
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 372.3	$ 412.2

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- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
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-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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+ References

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