Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,105.6	$ 1,807.5	$ 6,135.0	$ 5,188.3
Reimbursements	32.6	41.6	101.7	111.7
Total revenues	2,138.2	1,849.1	6,236.7	5,300.0
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,499.7	1,294.6	4,500.1	3,931.3
Total revenues	1,499.7	1,294.6	4,500.1	3,931.3
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	248.0	202.7	713.3	605.3
Reimbursements	32.6	41.6	101.7	111.7
Total revenues	280.6	244.3	815.0	717.0
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	357.9	310.2	921.6	651.7
Total revenues	357.9	310.2	921.6	651.7
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,016.2	889.9	3,118.7	2,734.6
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,016.2	889.9	3,118.7	2,734.6
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	601.7	495.8	1,696.4	1,462.8
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	353.8	293.2	983.4	858.1
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	247.9	202.6	713.0	604.7
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	61.0	54.7	183.0	164.0
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	61.0	54.7	183.0	164.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.7	34.5	150.3	117.0
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.7	34.5	150.3	117.0
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	20.8	19.0	56.0	53.6
Investment Income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	20.7	18.9	55.8	53.0
Investment Income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1	0.1	0.2	0.6
Net Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4.3	3.4	9.0	4.6
Net Gains on Divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4.3	3.4	8.9	4.6
Net Gains 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	310.8	919.2	652.1
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	357.6	310.8	919.2	652.1
Other Net Revenue (Losses) [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	(0.6)	2.4	(0.4)
Other Net Revenue (Losses) [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.3	$ (0.6)	$ 2.4	$ (0.4)