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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 1,825.2	$ 1,778.5	$ 5,473.6	$ 5,276.6
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,387.5	1,418.4	4,141.4	4,112.0
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	232.2	178.0	704.8	591.1
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	75.4	70.2	222.7	217.1
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	53.6	41.2	171.5	133.9
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	39.9	39.2	119.6	116.5
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	36.6	31.5	113.6	106.0
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,195.9	1,049.4	3,709.0	3,245.1
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	795.9	724.2	2,487.6	2,192.5
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	221.8	169.3	674.2	564.6
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	52.9	49.1	157.7	146.6
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	52.6	40.3	168.2	130.9
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	36.1	35.0	107.7	104.5
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	36.6	31.5	113.6	106.0
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	246.3	236.4	724.8	703.6
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	208.6	201.5	614.0	591.6
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	10.4	8.7	30.6	26.5
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	22.5	21.1	65.0	70.5
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.0	0.9	3.3	3.0
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.8	4.2	11.9	12.0
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	383.0	492.7	1,039.8	1,327.9
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 383.0	$ 492.7	$ 1,039.8	$ 1,327.9

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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://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

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+ References

No definition available.

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