Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,766.5	$ 2,453.6	$ 8,720.6	$ 7,533.4
Reimbursements	40.3	38.4	118.3	106.6
Total revenues	2,806.8	2,492.0	8,838.9	7,640.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,849.6	1,662.7	5,704.4	4,916.5
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	512.9	439.2	1,700.3	1,503.2
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	152.0	118.3	419.9	337.8
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	95.3	91.6	312.1	292.9
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	55.1	53.6	157.0	152.8
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	141.9	126.6	545.2	436.8
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2,396.4	2,122.1	7,637.6	6,585.7
Total revenues	2,396.4	2,122.1	7,637.6	6,585.7
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,516.6	1,349.0	4,725.2	4,024.0
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	497.2	427.3	1,656.6	1,468.4
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	93.8	79.8	252.7	227.8
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	93.7	89.8	307.1	288.1
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	53.2	49.6	150.8	140.6
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	141.9	126.6	545.2	436.8
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	369.7	331.0	1,081.1	947.2
Reimbursements	40.3	38.4	118.3	106.6
Total revenues	410.0	369.4	1,199.4	1,053.8
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	332.6	313.2	977.3	892.0
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	15.7	11.9	43.7	34.8
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	58.2	38.5	167.2	110.0
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.6	1.8	5.0	4.8
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.9	4.0	6.2	12.2
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.4	0.5	1.9	0.5
Total revenues	0.4	0.5	1.9	0.5
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.4	0.5	1.9	0.5
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,537.8	1,381.2	5,193.2	4,539.0
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,537.8	1,381.2	5,193.2	4,539.0
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	945.0	846.1	2,723.3	2,342.3
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	585.2	523.0	1,669.5	1,414.2
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	359.8	323.1	1,053.8	928.1
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	79.1	70.8	261.7	223.6
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	79.1	70.8	261.7	223.6
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	69.3	53.9	215.1	179.9
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	69.3	53.9	215.1	179.9
Interest Income Premium Finance Revenues and Other Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	135.3	101.6	327.3	248.6
Interest Income Premium Finance Revenues and Other Income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	125.0	93.2	298.1	229.0
Interest Income Premium Finance Revenues and Other Income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	9.9	7.9	27.3	19.1
Interest Income Premium Finance Revenues and Other Income [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.4	0.5	1.9	0.5
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 135.3	$ 101.6	$ 327.3	$ 248.6