Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 1,584.0	$ 1,657.8	$ 3,450.9	$ 3,648.4
Revenues before reimbursements	1,551.6	1,624.8	3,380.8	3,582.3
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,201.1	1,131.2	2,636.7	2,513.1
Revenues before reimbursements	1,201.1	1,131.2	2,636.7	2,513.1
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	223.2	242.1	472.7	478.5
Revenues before reimbursements	190.8	209.1	402.6	412.4
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	159.7	284.5	341.5	656.8
Revenues before reimbursements	159.7	284.5	341.5	656.8
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	827.5	777.7	1,844.7	1,718.1
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	827.5	777.7	1,844.7	1,718.1
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	459.7	464.7	967.0	929.4
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	269.1	256.1	564.9	517.9
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	190.6	208.6	402.1	411.5
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	50.3	46.9	109.3	103.6
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	50.3	46.9	109.3	103.6
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	37.4	29.5	82.5	77.5
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	37.4	29.5	82.5	77.5
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	16.0	19.6	34.6	37.9
Investment income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	15.8	19.1	34.1	37.0
Investment income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.2	0.5	0.5	0.9
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Total revenues	1.0	1.9	1.2	59.0
Net gains on divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.0	1.9	1.2	59.0
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	159.5	284.4	341.3	656.7
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	159.5	284.4	341.3	656.7
Other Net Losses [Member]
Segment Reporting Information [Line Items]
Total revenues	0.2	0.1	0.2	0.1
Other Net Losses [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.2	0.1	0.2	0.1
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	32.4	33.0	70.1	66.1
Reimbursements [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 32.4	$ 33.0	$ 70.1	$ 66.1