Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,008.4	$ 1,896.8	$ 4,412.9	$ 4,029.4
Reimbursements	34.6	36.7	65.4	69.1
Total revenues	2,043.0	1,933.5	4,478.3	4,098.5
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,740.7	1,390.2	3,863.3	3,000.4
Total revenues	1,740.7	1,390.2	3,863.3	3,000.4
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	267.4	245.0	526.5	465.3
Reimbursements	34.6	36.7	65.4	69.1
Total revenues	302.0	281.7	591.9	534.4
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	261.6	23.1	563.7
Total revenues	0.3	261.6	23.1	563.7
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,282.4	977.1	2,847.7	2,102.5
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,282.4	977.1	2,847.7	2,102.5
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	590.9	540.9	1,241.8	1,094.7
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	323.6	296.0	715.5	629.6
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	267.3	244.9	526.3	465.1
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	65.7	55.2	140.0	122.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	65.7	55.2	140.0	122.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.1	43.3	114.7	106.6
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.1	43.3	114.7	106.6
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	23.2	18.1	41.4	35.2
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	23.1	18.1	41.2	35.1
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1		0.2	0.1
Net Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2.8	0.6	4.2	4.7
Net Gains on Divestitures [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2.8	0.5	4.2	4.6
Net Gains on Divestitures [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		0.1		0.1
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	261.0	23.0	561.6
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.3	261.0	23.0	561.6
Other Net Revenue (Losses) [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		0.6	0.1	2.1
Other Net Revenue (Losses) [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		$ 0.6	$ 0.1	$ 2.1