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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 1,657.8	$ 1,660.4	$ 3,648.4	$ 3,498.1
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,176.7	1,235.8	2,754.0	2,693.6
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	258.7	213.2	472.6	413.1
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	78.8	79.6	147.3	146.9
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	59.5	49.0	117.9	92.8
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	45.3	43.7	79.6	77.3
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	38.8	39.1	77.0	74.4
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,131.2	1,000.1	2,513.1	2,195.7
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	685.3	615.5	1,691.8	1,468.3
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	248.7	204.4	452.4	395.4
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	58.8	53.9	104.8	97.5
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	58.4	47.8	115.6	90.6
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	41.2	39.4	71.5	69.5
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	38.8	39.1	77.0	74.4
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	242.1	237.3	478.5	467.2
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	206.9	197.3	405.4	390.1
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10.0	8.8	20.2	17.7
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	20.0	25.7	42.5	49.4
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.1	1.2	2.3	2.2
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	4.1	4.3	8.1	7.8
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	284.5	423.0	656.8	835.2
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 284.5	$ 423.0	$ 656.8	$ 835.2

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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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-Section 50
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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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