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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,736.0	$ 2,406.9	$ 5,954.1	$ 5,079.8
Reimbursements	39.4	35.0	78.0	68.2
Total revenues	2,775.4	2,441.9	6,032.1	5,148.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,719.9	1,498.3	3,854.8	3,253.8
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	588.6	539.2	1,187.4	1,064.0
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	146.9	121.3	267.9	219.5
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	112.0	106.1	216.8	201.3
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	58.0	57.3	101.9	99.2
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	150.0	119.7	403.3	310.2
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2,376.3	2,088.4	5,241.2	4,463.6
Total revenues	2,376.3	2,088.4	5,241.2	4,463.6
Brokerage [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,394.6	1,199.7	3,208.6	2,675.0
Brokerage [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	574.6	527.6	1,159.4	1,041.1
Brokerage [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	90.9	83.8	158.9	148.0
Brokerage [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	110.3	104.6	213.4	198.3
Brokerage [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	55.9	53.0	97.6	91.0
Brokerage [Member] | Other Foreign [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	150.0	119.7	403.3	310.2
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	358.6	318.6	711.4	616.2
Reimbursements	39.4	35.0	78.0	68.2
Total revenues	398.0	353.6	789.4	684.4
Risk Management [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	324.2	298.7	644.7	578.8
Risk Management [Member] | United Kingdom [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	14.0	11.6	28.0	22.9
Risk Management [Member] | Australia [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	56.0	37.5	109.0	71.5
Risk Management [Member] | Canada [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.7	1.5	3.4	3.0
Risk Management [Member] | New Zealand [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	2.1	4.3	4.3	8.2
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1.1	(0.1)	1.5
Total revenues	1.1	(0.1)	1.5
Corporate [Member] | Unites States [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.1	(0.1)	1.5
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,661.8	1,410.4	3,655.4	3,157.8
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,661.8	1,410.4	3,655.4	3,157.8
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	827.1	790.5	1,778.3	1,496.2
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	477.6	478.5	1,084.3	891.2
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	349.5	312.0	694.0	605.0
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	88.7	71.2	182.6	152.8
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	88.7	71.2	182.6	152.8
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	59.8	54.2	145.8	126.0
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	59.8	54.2	145.8	126.0
Interest Income Premium Finance Revenues and Other Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	98.6	80.6	192.0	147.0
Interest Income Premium Finance Revenues and Other Income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	88.4	74.1	173.1	135.8
Interest Income Premium Finance Revenues and Other Income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	9.1	6.6	17.4	11.2
Interest Income Premium Finance Revenues and Other Income [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1.1	(0.1)	1.5
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 98.6	$ 80.6	$ 192.0	$ 147.0

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