Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 2,435.3	$ 2,165.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,601.8	1,616.2
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	451.2	303.5
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	90.7	83.0
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	88.9	68.9
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	37.9	36.2
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	164.8	57.2
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	2,122.6	1,610.2
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,336.7	1,106.6
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	440.2	292.4
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	59.0	54.0
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	87.5	67.5
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	34.4	32.5
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	164.8	57.2
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	289.9	252.7
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	242.3	207.5
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	11.0	11.1
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	31.7	29.0
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.4	1.4
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.5	3.7
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	22.8	302.1
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 22.8	$ 302.1