Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Total revenues	$ 2,043.0	$ 1,933.5	$ 4,478.3	$ 4,098.5
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,265.5	1,305.5	2,867.3	2,921.7
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	438.7	337.9	889.9	641.4
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	110.9	103.7	201.6	186.7
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	95.2	81.3	184.1	150.2
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	50.8	49.4	88.7	85.6
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	81.9	55.7	246.7	112.9
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,740.7	1,390.2	3,863.3	3,000.4
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,015.0	812.4	2,351.7	1,919.0
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	426.2	326.1	866.4	618.5
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	76.6	70.7	135.6	124.7
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	93.8	79.9	181.3	147.4
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	47.2	45.4	81.6	77.9
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	81.9	55.7	246.7	112.9
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	302.0	281.7	591.9	534.4
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	250.2	231.5	492.5	439.0
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	12.5	11.8	23.5	22.9
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	34.3	33.0	66.0	62.0
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.4	1.4	2.8	2.8
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.6	4.0	7.1	7.7
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.3	261.6	23.1	563.7
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 0.3	$ 261.6	$ 23.1	$ 563.7