Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 1,849.1	$ 1,825.2	$ 5,300.0	$ 5,473.6
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,380.7	1,387.5	3,867.2	4,141.4
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	240.6	232.2	775.6	704.8
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	82.8	75.4	229.1	222.7
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	58.5	53.6	175.4	171.5
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	40.0	39.9	112.0	119.6
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	46.5	36.6	140.7	113.6
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,294.6	1,195.9	3,931.3	3,709.0
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	866.6	795.9	2,613.3	2,487.6
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	230.8	221.8	746.2	674.2
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	56.6	52.9	156.6	157.7
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	57.1	52.6	171.5	168.2
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	37.0	36.1	103.0	107.7
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	46.5	36.6	140.7	113.6
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	244.3	246.3	717.0	724.8
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	203.9	208.6	602.2	614.0
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	9.8	10.4	29.4	30.6
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	26.2	22.5	72.5	65.0
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.4	1.0	3.9	3.3
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.0	3.8	9.0	11.9
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	310.2	383.0	651.7	1,039.8
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 310.2	$ 383.0	$ 651.7	$ 1,039.8