Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 2,165.0	$ 1,866.9
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,616.2	1,400.4
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	303.5	265.9
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	83.0	64.7
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	68.9	57.2
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	36.2	30.8
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	57.2	47.9
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,610.2	1,435.6
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,106.6	1,005.9
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	292.4	254.5
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	54.0	43.1
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	67.5	56.2
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	32.5	28.0
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	57.2	47.9
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	252.7	249.5
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	207.5	212.7
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	11.1	11.4
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	29.0	21.6
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.4	1.0
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.7	2.8
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	302.1	181.8
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 302.1	$ 181.8