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Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 1,660.4	$ 1,489.5	[1]	$ 3,498.1	$ 3,135.9	[1]
Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	1,000.1	896.1		2,195.7	1,981.0
Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	237.3	216.8		467.2	426.7
Operating Segments [Member] | Corporate [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	423.0	376.6		835.2	728.2
Unites States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	1,235.8	1,114.0		2,693.6	2,441.0
Unites States [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	615.5	552.1		1,468.3	1,345.1
Unites States [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	197.3	185.3		390.1	367.7
Unites States [Member] | Operating Segments [Member] | Corporate [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	423.0	376.6		835.2	728.2
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	213.2	191.9		413.1	359.6
United Kingdom [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	204.4	184.7		395.4	346.0
United Kingdom [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	8.8	7.2		17.7	13.6
Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	79.6	71.1		146.9	130.1
Australia [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	53.9	52.0		97.5	92.3
Australia [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	25.7	19.1		49.4	37.8
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	49.0	37.5		92.8	70.4
Canada [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	47.8	36.6		90.6	68.7
Canada [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	1.2	0.9		2.2	1.7
New Zealand [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	43.7	41.1		77.3	69.8
New Zealand [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	39.4	36.8		69.5	63.9
New Zealand [Member] | Operating Segments [Member] | Risk Management [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	4.3	4.3		7.8	5.9
Other Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	39.1	33.9		74.4	65.0
Other Foreign [Member] | Operating Segments [Member] | Brokerage [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 39.1	$ 33.9		$ 74.4	$ 65.0
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

X

- 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
-Subparagraph (a)
-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
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

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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