Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1,624.8	$ 1,625.0	$ 3,582.3	$ 3,427.9
Total revenues	1,657.8	1,660.4	3,648.4	3,498.1
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,131.2	1,000.1	2,513.1	2,195.7
Total revenues	1,131.2	1,000.1	2,513.1	2,195.7
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	209.1	201.9	412.4	397.0
Total revenues	242.1	237.3	478.5	467.2
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	284.5	423.0	656.8	835.2
Total revenues	284.5	423.0	656.8	835.2
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	777.7	688.0	1,718.1	1,527.4
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	777.7	688.0	1,718.1	1,527.4
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	464.7	422.2	929.4	870.3
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	256.1	220.4	517.9	473.6
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	208.6	201.8	411.5	396.7
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	46.9	48.1	103.6	100.1
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	46.9	48.1	103.6	100.1
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	29.5	21.8	77.5	56.7
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	29.5	21.8	77.5	56.7
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	19.6	15.8	37.9	29.2
Investment income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	19.1	15.7	37.0	28.9
Investment income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.5	0.1	0.9	0.3
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Total revenues	1.9	6.1	59.0	9.0
Net gains on divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.9	6.1	59.0	9.0
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	284.4	422.4	656.7	834.6
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	284.4	422.4	656.7	834.6
Other net gains [Member]
Segment Reporting Information [Line Items]
Total revenues	0.1	0.6	0.1	0.6
Other net gains [Member] \| Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	0.1	0.6	0.1	0.6
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	33.0	35.4	66.1	70.2
Reimbursements [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 33.0	$ 35.4	$ 66.1	$ 70.2