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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1,790.5	$ 1,741.2	$ 5,372.8	$ 5,169.1
Total revenues	1,825.2	1,778.5	5,473.6	5,276.6
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,195.9	1,049.4	3,709.0	3,245.1
Total revenues	1,195.9	1,049.4	3,709.0	3,245.1
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	211.6	199.1	624.0	596.1
Total revenues	246.3	236.4	724.8	703.6
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	383.0	492.7	1,039.8	1,327.9
Total revenues	383.0	492.7	1,039.8	1,327.9
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	809.9	707.6	2,528.0	2,235.0
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	809.9	707.6	2,528.0	2,235.0
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	490.1	452.7	1,419.5	1,323.0
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	278.9	253.7	796.8	727.3
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	211.2	199.0	622.7	595.7
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	49.8	43.9	153.4	144.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	49.8	43.9	153.4	144.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	30.4	25.7	107.9	82.4
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	30.4	25.7	107.9	82.4
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	24.0	18.0	61.9	47.2
Investment income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	23.6	17.9	60.6	46.8
Investment income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.4	0.1	1.3	0.4
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Total revenues	3.3	0.6	62.3	9.6
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.3	0.6	62.3	9.6
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	386.0	492.4	1,042.7	1,327.0
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	386.0	492.4	1,042.7	1,327.0
Other net gains [Member]
Segment Reporting Information [Line Items]
Total revenues	(3.0)	0.3	(2.9)	0.9
Other net gains [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues	(3.0)	0.3	(2.9)	0.9
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	34.7	37.3	100.8	107.5
Reimbursements [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 34.7	$ 37.3	$ 100.8	$ 107.5

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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