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Segment Information - Summary of Revenues by Type and Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1,957.5	$ 1,802.9
Total revenues	1,990.6	1,837.7
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,381.9	1,195.6
Total revenues	1,381.9	1,195.6
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	203.3	195.1
Total revenues	236.4	229.9
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	372.3	412.2
Total revenues	372.3	412.2
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues	940.4	839.4
Commissions [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	940.4	839.4
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues	464.7	448.1
Broker Fees [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	261.8	253.2
Broker Fees [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	202.9	194.9
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues	56.7	52.0
Supplemental Revenue Member [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	56.7	52.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	48.0	34.9
Contingent Revenue [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	48.0	34.9
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues	18.3	13.4
Investment income [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	17.9	13.2
Investment income [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	0.4	0.2
Net gains on divestitures [Member]
Segment Reporting Information [Line Items]
Total revenues	57.1	2.9
Net gains on divestitures [Member] | Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	57.1	2.9
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues	372.3	412.2
Clean Coal Activities [Member] | Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	372.3	412.2
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues	33.1	34.8
Reimbursements [Member] | Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 33.1	$ 34.8

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