Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,406.9	$ 2,008.4	$ 5,079.8	$ 4,412.9
Reimbursements	35.0	34.6	68.2	65.4
Total revenues	2,441.9	2,043.0	5,148.0	4,478.3
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2,088.4	1,740.7	4,463.6	3,863.3
Total revenues	2,088.4	1,740.7	4,463.6	3,863.3
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	318.6	267.4	616.2	526.5
Reimbursements	35.0	34.6	68.2	65.4
Total revenues	353.6	302.0	684.4	591.9
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	(0.1)	0.3		23.1
Total revenues	(0.1)	0.3		23.1
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,410.4	1,282.4	3,157.8	2,847.7
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,410.4	1,282.4	3,157.8	2,847.7
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	790.5	590.9	1,496.2	1,241.8
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	478.5	323.6	891.2	715.5
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	312.0	267.3	605.0	526.3
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	71.2	65.7	152.8	140.0
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	71.2	65.7	152.8	140.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	54.2	43.1	126.0	114.7
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	54.2	43.1	126.0	114.7
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	75.6	23.2	141.6	41.4
Investment Income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	69.1	23.1	130.6	41.2
Investment Income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	6.5	0.1	11.0	0.2
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	5.1	2.8	5.4	4.2
Net gains on divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	5.0	2.8	5.2	4.2
Net gains on divestitures [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1		$ 0.2
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		0.3		23.0
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		$ 0.3		23.0
Other Net Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	(0.1)			0.1
Other Net Revenue [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ (0.1)			$ 0.1