Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,012.0	$ 2,105.6	$ 6,424.9	$ 6,135.0
Reimbursements	32.0	32.6	97.4	101.7
Total revenues	2,044.0	2,138.2	6,522.3	6,236.7
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,736.2	1,499.7	5,599.5	4,500.1
Total revenues	1,736.2	1,499.7	5,599.5	4,500.1
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	275.5	248.0	802.0	713.3
Reimbursements	32.0	32.6	97.4	101.7
Total revenues	307.5	280.6	899.4	815.0
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	357.9	23.4	921.6
Total revenues	0.3	357.9	23.4	921.6
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,186.9	1,016.2	4,034.6	3,118.7
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,186.9	1,016.2	4,034.6	3,118.7
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	671.3	601.7	1,913.1	1,696.4
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	396.0	353.8	1,111.5	983.4
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	275.3	247.9	801.6	713.0
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	64.7	61.0	204.7	183.0
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	64.7	61.0	204.7	183.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	52.4	43.7	167.1	150.3
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	52.4	43.7	167.1	150.3
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	37.7	20.8	79.1	56.0
Investment Income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	37.5	20.7	78.7	55.8
Investment Income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.2	0.1	0.4	0.2
Net losses on divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	(1.3)	4.3	2.9	9.0
Net losses on divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	(1.3)	4.3	2.9	8.9
Net losses on divestitures [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements				0.1
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		357.6	23.0	919.2
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		357.6	23.0	919.2
Other Net Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	0.3	0.4	2.4
Other Net Revenue [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.3	$ 0.3	$ 0.4	$ 2.4