Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1,896.8	$ 1,551.6	$ 4,029.4	$ 3,380.8
Reimbursements	36.7	32.4	69.1	70.1
Total revenues	1,933.5	1,584.0	4,098.5	3,450.9
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,390.2	1,201.1	3,000.4	2,636.7
Total revenues	1,390.2	1,201.1	3,000.4	2,636.7
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	245.0	190.8	465.3	402.6
Reimbursements	36.7	32.4	69.1	70.1
Total revenues	281.7	223.2	534.4	472.7
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	261.6	159.7	563.7	341.5
Total revenues	261.6	159.7	563.7	341.5
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	977.1	827.5	2,102.5	1,844.7
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	977.1	827.5	2,102.5	1,844.7
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	540.9	459.7	1,094.7	967.0
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	296.0	269.1	629.6	564.9
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	244.9	190.6	465.1	402.1
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	55.2	50.3	122.0	109.3
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	55.2	50.3	122.0	109.3
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.3	37.4	106.6	82.5
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	43.3	37.4	106.6	82.5
Investment Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	18.1	16.0	35.2	34.6
Investment Income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	18.1	15.8	35.1	34.1
Investment Income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		0.2	0.1	0.5
Net Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.6	1.0	4.7	1.2
Net Gains on Divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.5	1.0	4.6	1.2
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	261.0	159.5	561.6	341.3
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	261.0	159.5	561.6	341.3
Other Net Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.6	0.2	2.1	0.2
Other Net Revenue [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.6	$ 0.2	$ 2.1	$ 0.2