Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 9,926.5	$ 8,420.1	$ 8,076.4
Reimbursements	145.4	130.5	133.0
Total revenues	10,071.9	8,550.6	8,209.4
Compensation	5,681.2	4,799.8	3,927.5
Operating	1,689.7	1,330.9	1,072.4
Reimbursements	145.4	130.5	133.0
Cost of revenues from clean coal activities		22.9	1,173.2
Interest	296.7	256.9	226.1
Loss on extinguishment of debt			16.2
Depreciation	165.2	144.7	151.2
Amortization	531.3	454.9	415.1
Change in estimated acquisition earnout payables	377.3	83.0	119.6
Total expenses	8,886.8	7,223.6	7,234.3
Earnings before income taxes	1,185.1	1,327.0	975.1
Provision (benefit) for income taxes	219.1	211.0	20.1
Net earnings	966.0	1,116.0	955.0
Net earnings (loss) attributable to noncontrolling interests	(3.5)	1.8	48.2
Net earnings attributable to controlling interests	969.5	1,114.2	906.8
Net foreign exchange (loss) gain	(10.3)	34.0	(2.3)
Total identifiable assets	51,615.8	38,358.4	33,236.1
Goodwill - net	11,475.6	9,489.4	8,666.2
Amortizable intangible assets - net	4,633.3	3,372.1	3,954.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	6,426.5	5,557.2	5,856.8
Total identifiable assets	25,310.3	20,940.6	18,559.3
Goodwill - net	6,379.3	5,140.5
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	1,994.1	1,588.4	1,245.0
Total identifiable assets	16,129.6	9,454.4	8,304.8
Goodwill - net	2,511.9	2,197.9
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	466.8	410.9	376.1
Total identifiable assets	2,438.9	1,881.1	1,759.0
Goodwill - net	650.5	477.8
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	403.9	361.9	308.0
Total identifiable assets	1,697.0	1,469.7	1,431.6
Goodwill - net	623.7	569.7
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	208.0	181.3	177.0
Total identifiable assets	793.2	749.7	774.9
Goodwill - net	213.8	213.3
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	572.6	450.9	246.5
Total identifiable assets	5,246.8	3,862.9	2,406.5
Goodwill - net	1,096.4	890.2
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	5,865.0	5,187.4	4,132.3
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	3,144.7	2,567.7	2,264.1
Supplemental Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	314.2	284.7	248.7
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	235.3	207.3	188.0
Interest Income, Premium Finance Revenues and Other Income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	367.3	150.0	102.5
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		23.0	1,140.8
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	11,217.8	9,358.1	8,544.6
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	8,637.2	7,303.8	5,967.5
Total revenues	8,637.2	7,303.8	5,967.5
Compensation	4,769.1	4,024.7	3,252.4
Operating	1,272.3	1,039.9	757.9
Depreciation	124.4	103.6	87.8
Amortization	523.6	448.7	407.6
Change in estimated acquisition earnout payables	376.8	90.4	116.3
Total expenses	7,066.2	5,707.3	4,622.0
Earnings before income taxes	1,571.0	1,596.5	1,345.5
Provision (benefit) for income taxes	401.6	394.7	328.9
Net earnings	1,169.4	1,201.8	1,016.6
Net earnings (loss) attributable to noncontrolling interests	6.3	4.4	8.4
Net earnings attributable to controlling interests	1,163.1	1,197.4	1,008.2
Net foreign exchange (loss) gain	(0.3)	2.6	(1.7)
Total identifiable assets	47,446.1	34,675.0	29,569.7
Goodwill - net	11,217.8	9,358.1	8,544.6
Amortizable intangible assets - net	4,427.9	3,325.9	3,906.1
Brokerage [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	5,216.1	4,503.9	3,804.8
Total identifiable assets	21,763.9	17,485.3	15,136.5
Goodwill - net	6,304.5	5,065.7
Brokerage [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	1,946.5	1,544.3	1,199.0
Total identifiable assets	15,999.7	9,338.5	8,170.7
Goodwill - net	2,493.4	2,180.2
Brokerage [Member] | Operating Segments [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	312.1	281.8	252.6
Total identifiable assets	1,969.7	1,792.1	1,674.3
Goodwill - net	514.6	467.6
Brokerage [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	397.7	356.0	302.2
Total identifiable assets	1,692.9	1,465.3	1,427.2
Goodwill - net	623.7	569.7
Brokerage [Member] | Operating Segments [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	192.2	166.9	162.4
Total identifiable assets	773.1	730.9	754.5
Goodwill - net	204.2	203.8
Brokerage [Member] | Operating Segments [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	572.6	450.9	246.5
Total identifiable assets	5,246.8	3,862.9	2,406.5
Goodwill - net	1,077.4	871.1
Brokerage [Member] | Commissions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	5,865.0	5,187.4	4,132.3
Brokerage [Member] | Broker Fees [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,885.0	1,476.9	1,296.9
Brokerage [Member] | Supplemental Revenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	314.2	284.7	248.7
Brokerage [Member] | Contingent Revenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	235.3	207.3	188.0
Brokerage [Member] | Interest Income, Premium Finance Revenues and Other Income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	337.7	147.5	101.6
Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	238.8	112.2	100.9
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,287.6	1,092.6	967.6
Reimbursements	145.4	130.5	133.0
Total revenues	1,433.0	1,223.1	1,100.6
Compensation	776.8	664.9	580.7
Operating	257.4	233.9	209.8
Reimbursements	145.4	130.5	133.0
Depreciation	35.9	37.8	46.2
Amortization	7.7	6.2	7.5
Change in estimated acquisition earnout payables	0.5	(7.4)	3.3
Total expenses	1,223.7	1,065.9	980.5
Earnings before income taxes	209.3	157.2	120.1
Provision (benefit) for income taxes	55.3	41.4	30.6
Net earnings	154.0	115.8	89.5
Net earnings attributable to controlling interests	154.0	115.8	89.5
Net foreign exchange (loss) gain	(9.9)	31.4
Total identifiable assets	1,649.3	1,142.6	1,033.8
Goodwill - net	238.8	112.2	100.9
Amortizable intangible assets - net	205.4	46.2	47.9
Risk Management [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,208.7	1,029.6	910.7
Total identifiable assets	1,026.0	914.5	790.2
Goodwill - net	74.8	74.8
Risk Management [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	47.6	44.1	46.0
Total identifiable assets	129.9	115.9	134.1
Goodwill - net	18.5	17.7
Risk Management [Member] | Operating Segments [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	154.7	129.1	123.5
Total identifiable assets	469.2	89.0	84.7
Goodwill - net	135.9	10.2
Risk Management [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	6.2	5.9	5.8
Total identifiable assets	4.1	4.4	4.4
Risk Management [Member] | Operating Segments [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	15.8	14.4	14.6
Total identifiable assets	20.1	18.8	20.4
Goodwill - net	9.6	9.5
Risk Management [Member] | Broker Fees [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1,259.7	1,090.8	967.2
Risk Management [Member] | Interest Income, Premium Finance Revenues and Other Income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	27.9	1.8	0.4
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	19.0	19.1	20.7
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	1.7	23.7	1,141.3
Total revenues	1.7	23.7	1,141.3
Compensation	135.3	110.2	94.4
Operating	160.0	57.1	104.7
Cost of revenues from clean coal activities		22.9	1,173.2
Interest	296.7	256.9	226.1
Loss on extinguishment of debt			16.2
Depreciation	4.9	3.3	17.2
Total expenses	596.9	450.4	1,631.8
Earnings before income taxes	(595.2)	(426.7)	(490.5)
Provision (benefit) for income taxes	(237.8)	(225.1)	(339.4)
Net earnings	(357.4)	(201.6)	(151.1)
Net earnings (loss) attributable to noncontrolling interests	(9.8)	(2.6)	39.8
Net earnings attributable to controlling interests	(347.6)	(199.0)	(190.9)
Net foreign exchange (loss) gain	(0.1)		(0.6)
Total identifiable assets	2,520.4	2,540.8	2,632.6
Goodwill - net	19.0	19.1	20.7
Corporate [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1.7	23.7	1,141.3
Total identifiable assets	2,520.4	2,540.8	2,632.6
Corporate [Member] | Operating Segments [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	19.0	19.1
Corporate [Member] | Interest Income, Premium Finance Revenues and Other Income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements	$ 1.7	0.7	0.5
Corporate [Member] | Clean Coal Activities [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements		$ 23.0	$ 1,140.8