Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 1,849.1	$ 1,825.2	$ 5,300.0	$ 5,473.6
Revenues before reimbursements	1,807.5	1,790.5	5,188.3	5,372.8
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,294.6	1,195.9	3,931.3	3,709.0
Revenues before reimbursements	1,294.6	1,195.9	3,931.3	3,709.0
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	244.3	246.3	717.0	724.8
Revenues before reimbursements	202.7	211.6	605.3	624.0
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	310.2	383.0	651.7	1,039.8
Revenues before reimbursements	310.2	383.0	651.7	1,039.8
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	889.9	809.9	2,734.6	2,528.0
Commissions [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	889.9	809.9	2,734.6	2,528.0
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	495.8	490.1	1,462.8	1,419.5
Broker Fees [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	293.2	278.9	858.1	796.8
Broker Fees [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	202.6	211.2	604.7	622.7
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	54.7	49.8	164.0	153.4
Supplemental Revenue Member [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	54.7	49.8	164.0	153.4
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	34.5	30.4	117.0	107.9
Contingent Revenue [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	34.5	30.4	117.0	107.9
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	19.0	24.0	53.6	61.9
Investment income [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	18.9	23.6	53.0	60.6
Investment income [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.1	0.4	0.6	1.3
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Total revenues	3.4	3.3	4.6	62.3
Net gains on divestitures [Member] \| Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.4	3.3	4.6	62.3
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	310.8	386.0	652.1	1,042.7
Clean Coal Activities [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	310.8	386.0	652.1	1,042.7
Other Net Losses [Member]
Segment Reporting Information [Line Items]
Total revenues	(0.6)	(3.0)	(0.4)	(2.9)
Other Net Losses [Member] \| Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	(0.6)	(3.0)	(0.4)	(2.9)
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	41.6	34.7	111.7	100.8
Reimbursements [Member] \| Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 41.6	$ 34.7	$ 111.7	$ 100.8