Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 8,076.4	$ 6,851.9	$ 7,056.4
Reimbursements	133.0	151.7	138.6
Total revenues	8,209.4	7,003.6	7,195.0
Compensation	3,927.5	3,466.5	3,339.5
Operating	1,072.4	906.5	1,068.5
Reimbursements	133.0	151.7	138.6
Cost of revenues from clean coal activities	1,173.2	882.1	1,352.8
Interest	226.1	196.4	179.8
Loss on extinguishment of debt	16.2
Depreciation	151.2	145.1	140.4
Amortization	415.1	417.3	334.0
Change in estimated acquisition earnout payables	119.6	(32.9)	15.3
Total expenses	7,234.3	6,132.7	6,568.9
Earnings (loss) before income taxes	975.1	870.9	626.1
Provision (benefit) for income taxes	20.1	12.8	(89.7)
Net earnings (loss)	955.0	858.1	715.8
Net earnings attributable to noncontrolling interests	48.2	39.3	47.0
Net earnings (loss) attributable to controlling interests	906.8	818.8	668.8
Net foreign exchange (loss) gain	(2.3)	(2.9)	(6.7)
Total identifiable assets	33,345.0	22,331.4	19,634.8
Goodwill - net	8,666.2	6,127.0	5,618.5
Amortizable intangible assets - net	3,954.0	2,399.9	2,318.7
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	5,856.8	5,048.9	5,379.1
Total identifiable assets	18,828.6	11,786.3	10,782.7
Goodwill - net	4,473.9	3,433.0
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	1,245.0	1,034.4	963.4
Total identifiable assets	8,385.4	6,274.3	5,091.1
Goodwill - net	2,213.7	1,343.4
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	376.1	314.5	298.6
Total identifiable assets	1,768.9	1,462.9	1,307.9
Goodwill - net	498.1	473.6
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	308.0	249.1	226.0
Total identifiable assets	1,440.5	1,058.4	916.7
Goodwill - net	572.9	492.9
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	177.0	154.4	160.8
Total identifiable assets	775.0	790.1	718.7
Goodwill - net	227.8	232.6
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	246.5	202.3	167.1
Total identifiable assets	2,146.6	959.4	817.7
Goodwill - net	679.8	151.5
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4,132.3	3,591.9	3,320.6
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	2,264.1	1,957.9	1,911.1
Supplemental Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	248.7	221.9	210.5
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	188.0	147.0	135.6
Investment income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	83.1	75.9	86.9
Net (Losses) Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	18.9	(5.8)	75.3
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,140.8	863.5	1,319.3
Other Net Revenue (Losses) [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.5	(0.4)	(2.9)
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	8,544.6	6,053.6	5,548.9
Brokerage [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	4,409.1	3,399.8
Brokerage [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2,198.7	1,328.3
Brokerage [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	487.2	462.1
Brokerage [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Goodwill - net	572.9	492.9
Brokerage [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	217.6	221.9
Brokerage [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	659.1	148.6
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	5,967.5	5,167.1	4,901.5
Total revenues	5,967.5	5,167.1	4,901.5
Compensation	3,252.4	2,882.5	2,745.9
Operating	757.9	687.2	796.5
Depreciation	87.8	73.5	66.6
Amortization	407.6	411.3	329.1
Change in estimated acquisition earnout payables	116.3	(29.7)	16.9
Total expenses	4,622.0	4,024.8	3,955.0
Earnings (loss) before income taxes	1,345.5	1,142.3	946.5
Provision (benefit) for income taxes	328.9	276.3	229.2
Net earnings (loss)	1,016.6	866.0	717.3
Net earnings attributable to noncontrolling interests	8.4	4.9	17.2
Net earnings (loss) attributable to controlling interests	1,008.2	861.1	700.1
Net foreign exchange (loss) gain	(1.7)	(2.6)	(1.0)
Total identifiable assets	29,821.0	19,185.3	16,741.9
Goodwill - net	8,544.6	6,053.6	5,548.9
Amortizable intangible assets - net	3,906.1	2,376.3	2,289.9
Brokerage [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	3,804.8	3,369.4	3,234.3
Total identifiable assets	15,548.3	8,897.9	8,132.3
Brokerage [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	1,199.0	993.7	921.8
Total identifiable assets	8,251.3	6,135.1	4,964.5
Brokerage [Member] | Operating Segments [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	252.6	216.1	211.3
Total identifiable assets	1,684.2	1,373.3	1,217.9
Brokerage [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	302.2	243.8	221.4
Total identifiable assets	1,436.1	1,053.6	913.6
Brokerage [Member] | Operating Segments [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	162.4	141.8	145.6
Total identifiable assets	754.5	766.0	695.9
Brokerage [Member] | Operating Segments [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	246.5	202.3	167.1
Total identifiable assets	2,146.6	959.4	817.7
Brokerage [Member] | Commissions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	4,132.3	3,591.9	3,320.6
Brokerage [Member] | Broker Fees [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,296.9	1,136.9	1,074.2
Brokerage [Member] | Supplemental Revenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	248.7	221.9	210.5
Brokerage [Member] | Contingent Revenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	188.0	147.0	135.6
Brokerage [Member] | Investment income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	82.8	75.2	85.3
Brokerage [Member] | Net (Losses) Gains on Divestitures [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	18.8	(5.8)	75.3
Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	100.9	70.5	66.6
Risk Management [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	64.8	33.2
Risk Management [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	15.0	15.1
Risk Management [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	10.9	11.5
Risk Management [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	10.2	10.7
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	967.6	821.7	838.5
Reimbursements	133.0	151.7	138.6
Total revenues	1,100.6	973.4	977.1
Compensation	580.7	517.5	515.7
Operating	209.8	162.6	184.9
Reimbursements	133.0	151.7	138.6
Depreciation	46.2	49.4	46.2
Amortization	7.5	6.0	4.9
Change in estimated acquisition earnout payables	3.3	(3.2)	(1.6)
Total expenses	980.5	884.0	888.7
Earnings (loss) before income taxes	120.1	89.4	88.4
Provision (benefit) for income taxes	30.6	22.5	22.2
Net earnings (loss)	89.5	66.9	66.2
Net earnings (loss) attributable to controlling interests	89.5	66.9	66.2
Net foreign exchange (loss) gain		(0.1)	(0.1)
Total identifiable assets	1,034.4	973.9	898.1
Goodwill - net	100.9	70.5	66.6
Amortizable intangible assets - net	47.9	23.6	28.8
Risk Management [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	910.7	816.4	828.4
Total identifiable assets	790.7	716.2	655.6
Risk Management [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	46.0	40.7	41.6
Total identifiable assets	134.1	139.2	126.6
Risk Management [Member] | Operating Segments [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	123.5	98.4	87.3
Total identifiable assets	84.7	89.6	90.0
Risk Management [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	5.8	5.3	4.6
Total identifiable assets	4.4	4.8	3.1
Risk Management [Member] | Operating Segments [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	14.6	12.6	15.2
Total identifiable assets	20.5	24.1	22.8
Risk Management [Member] | Broker Fees [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	967.2	821.0	836.9
Risk Management [Member] | Investment income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.3	0.7	1.6
Risk Management [Member] | Net (Losses) Gains on Divestitures [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	0.1
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	20.7	2.9	3.0
Corporate [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	20.7	2.9
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,141.3	863.1	1,316.4
Total revenues	1,141.3	863.1	1,316.4
Compensation	94.4	66.5	77.9
Operating	104.7	56.7	87.1
Cost of revenues from clean coal activities	1,173.2	882.1	1,352.8
Interest	226.1	196.4	179.8
Loss on extinguishment of debt	16.2
Depreciation	17.2	22.2	27.6
Total expenses	1,631.8	1,223.9	1,725.2
Earnings (loss) before income taxes	(490.5)	(360.8)	(408.8)
Provision (benefit) for income taxes	(339.4)	(286.0)	(341.1)
Net earnings (loss)	(151.1)	(74.8)	(67.7)
Net earnings attributable to noncontrolling interests	39.8	34.4	29.8
Net earnings (loss) attributable to controlling interests	(190.9)	(109.2)	(97.5)
Net foreign exchange (loss) gain	(0.6)	(0.2)	(5.6)
Total identifiable assets	2,489.6	2,172.2	1,994.8
Goodwill - net	20.7	2.9	3.0
Corporate [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,141.3	863.1	1,316.4
Total identifiable assets	2,489.6	2,172.2	1,994.8
Corporate [Member] | Clean Coal Activities [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,140.8	863.5	1,319.3
Corporate [Member] | Other Net Revenue (Losses) [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 0.5	$ (0.4)	$ (2.9)