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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, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1,741.2	$ 1,559.9	[1]	$ 5,169.1	$ 4,627.5	[1]
Total revenues	1,778.5	1,593.7	[1]	5,276.6	4,729.6	[1]
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	707.6	637.9	[1]	2,235.0	2,016.8	[1]
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	452.7	416.2	[1]	1,323.0	1,208.1	[1]
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	43.9	36.9	[1]	144.0	120.0	[1]
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	25.7	21.8	[1]	82.4	78.1	[1]
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	18.0	15.9	[1]	47.2	42.6	[1]
Gains on Books of Business Sales [Member]
Segment Reporting Information [Line Items]
Total revenues	0.6	0.6	[1]	9.6	3.1	[1]
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	492.4	430.6	[1]	1,327.0	1,158.8	[1]
Product and Service, Other [Member]
Segment Reporting Information [Line Items]
Total revenues	0.3			0.9
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	37.3	33.8	[1]	107.5	102.1	[1]
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,049.4	938.7		3,245.1	2,919.7
Total revenues	1,049.4	938.7		3,245.1	2,919.7
Operating Segments [Member] | Brokerage [Member] | Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	707.6	637.9		2,235.0	2,016.8
Operating Segments [Member] | Brokerage [Member] | Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	253.7	225.7		727.3	659.5
Operating Segments [Member] | Brokerage [Member] | Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	43.9	36.9		144.0	120.0
Operating Segments [Member] | Brokerage [Member] | Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	25.7	21.8		82.4	78.1
Operating Segments [Member] | Brokerage [Member] | Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	17.9	15.8		46.8	42.2
Operating Segments [Member] | Brokerage [Member] | Gains on Books of Business Sales [Member]
Segment Reporting Information [Line Items]
Total revenues	0.6	0.6		9.6	3.1
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	199.1	190.6		596.1	549.0
Total revenues	236.4	224.4		703.6	651.1
Operating Segments [Member] | Risk Management [Member] | Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	199.0	190.5		595.7	548.6
Operating Segments [Member] | Risk Management [Member] | Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	0.1	0.1		0.4	0.4
Operating Segments [Member] | Risk Management [Member] | Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	37.3	33.8		107.5	102.1
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	492.7	430.6		1,327.9	1,158.8
Total revenues	492.7	430.6		1,327.9	1,158.8
Operating Segments [Member] | Corporate [Member] | Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	492.4	$ 430.6		1,327.0	$ 1,158.8
Operating Segments [Member] | Corporate [Member] | Product and Service, Other [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 0.3			$ 0.9
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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