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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 2,132.6	$ 1,829.2
Reimbursements	32.4	37.7
Total revenues	2,165.0	1,866.9
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,610.2	1,435.6
Total revenues	1,610.2	1,435.6
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	220.3	211.8
Reimbursements	32.4	37.7
Total revenues	252.7	249.5
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	302.1	181.8
Total revenues	302.1	181.8
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,125.4	1,017.2
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,125.4	1,017.2
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	553.8	507.3
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	333.6	295.8
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	220.2	211.5
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	66.8	59.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	66.8	59.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	63.3	45.1
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	63.3	45.1
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	17.1	18.6
Investment Income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	17.0	18.3
Investment Income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1	0.3
Net Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4.1	0.2
Net Gains on Divestitures [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4.1	0.2
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	300.6	181.8
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	300.6	$ 181.8
Other Net Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1.5
Other Net Revenue [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1.5

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