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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1,625.0	$ 1,454.3	[1]	$ 3,427.9	$ 3,067.6	[1]
Total revenues	1,660.4	1,489.5	[1]	3,498.1	3,135.9	[1]
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	688.0	618.2	[1]	1,527.4	1,378.9	[1]
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	422.2	386.9	[1]	870.3	791.9	[1]
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	48.1	35.8	[1]	100.1	83.1	[1]
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	21.8	21.3	[1]	56.7	56.3	[1]
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	15.8	14.4	[1]	29.2	26.7	[1]
Gains on Books of Business Sales [Member]
Segment Reporting Information [Line Items]
Total revenues	6.1	1.1	[1]	9.0	2.5	[1]
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	422.4	376.4	[1]	834.6	728.2	[1]
Product and Service, Other [Member]
Segment Reporting Information [Line Items]
Total revenues	0.6	0.2	[1]	0.6
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	35.4	35.2	[1]	70.2	68.3	[1]
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,000.1	896.1		2,195.7	1,981.0
Total revenues	1,000.1	896.1		2,195.7	1,981.0
Operating Segments [Member] | Brokerage [Member] | Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	688.0	618.2		1,527.4	1,378.9
Operating Segments [Member] | Brokerage [Member] | Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	220.4	205.5		473.6	433.8
Operating Segments [Member] | Brokerage [Member] | Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	48.1	35.8		100.1	83.1
Operating Segments [Member] | Brokerage [Member] | Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	21.8	21.3		56.7	56.3
Operating Segments [Member] | Brokerage [Member] | Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	15.7	14.2		28.9	26.4
Operating Segments [Member] | Brokerage [Member] | Gains on Books of Business Sales [Member]
Segment Reporting Information [Line Items]
Total revenues	6.1	1.1		9.0	2.5
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	201.9	181.6		397.0	358.4
Total revenues	237.3	216.8		467.2	426.7
Operating Segments [Member] | Risk Management [Member] | Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	201.8	181.4		396.7	358.1
Operating Segments [Member] | Risk Management [Member] | Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	0.1	0.2		0.3	0.3
Operating Segments [Member] | Risk Management [Member] | Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	35.4	35.2		70.2	68.3
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	423.0	376.6		835.2	728.2
Total revenues	423.0	376.6		835.2	728.2
Operating Segments [Member] | Corporate [Member] | Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	422.4	376.4		834.6	$ 728.2
Operating Segments [Member] | Corporate [Member] | Product and Service, Other [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 0.6	$ 0.2		$ 0.6
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to ASC 606.

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