Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,453.6	$ 2,012.0	$ 7,533.4	$ 6,424.9
Reimbursements	38.4	32.0	106.6	97.4
Total revenues	2,492.0	2,044.0	7,640.0	6,522.3
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2,122.1	1,736.2	6,585.7	5,599.5
Total revenues	2,122.1	1,736.2	6,585.7	5,599.5
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	331.0	275.5	947.2	802.0
Reimbursements	38.4	32.0	106.6	97.4
Total revenues	369.4	307.5	1,053.8	899.4
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.5	0.3	0.5	23.4
Total revenues	0.5	0.3	0.5	23.4
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,381.2	1,186.9	4,539.0	4,034.6
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,381.2	1,186.9	4,539.0	4,034.6
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	846.1	671.3	2,342.3	1,913.1
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	523.0	396.0	1,414.2	1,111.5
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	323.1	275.3	928.1	801.6
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	70.8	64.7	223.6	204.7
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	70.8	64.7	223.6	204.7
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	53.9	52.4	179.9	167.1
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	53.9	52.4	179.9	167.1
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	100.6	37.7	242.2	79.1
Investment Income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	92.8	37.5	223.4	78.7
Investment Income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	7.8	0.2	18.8	0.4
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.5	(1.3)	5.9	2.9
Net gains on divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.4	(1.3)	5.6	2.9
Net gains on divestitures [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1		0.3
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements				23.0
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements				23.0
Other Net Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.5	0.3	0.5	0.4
Other Net Revenue [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.5	$ 0.3	$ 0.5	$ 0.4