Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,404.5	$ 2,132.6
Reimbursements	30.8	32.4
Total revenues	2,435.3	2,165.0
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2,122.6	1,610.2
Total revenues	2,122.6	1,610.2
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	259.1	220.3
Reimbursements	30.8	32.4
Total revenues	289.9	252.7
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	22.8	302.1
Total revenues	22.8	302.1
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,565.3	1,125.4
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,565.3	1,125.4
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	650.9	553.8
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	391.9	333.6
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	259.0	220.2
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	74.3	66.8
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	74.3	66.8
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	71.6	63.3
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	71.6	63.3
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	18.2	17.1
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	18.1	17.0
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1	0.1
Net Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1.4	4.1
Net Gains on Divestitures [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1.4	4.1
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	22.7	300.6
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	22.7	300.6
Other Net Revenue (Losses) [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1	1.5
Other Net Revenue (Losses) [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.1	$ 1.5