Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues before reimbursements									$ 6,851.9	$ 7,056.4	$ 6,792.4
Reimbursements									151.7	138.6	141.6
Total revenues	$ 1,703.6	$ 1,849.1	$ 1,584.0	$ 1,866.9	$ 1,721.4	$ 1,825.2	$ 1,657.8	$ 1,990.6	7,003.6	7,195.0	6,934.0
Compensation									3,466.5	3,339.5	3,026.3
Operating									906.5	1,068.5	903.7
Reimbursements									151.7	138.6	141.6
Cost of revenues from clean coal activities									882.1	1,352.8	1,816.0
Interest									196.4	179.8	138.4
Depreciation									145.1	140.4	127.8
Amortization									417.3	334.0	291.2
Change in estimated acquisition earnout payables									(32.9)	15.3	9.6
Total expenses	1,569.9	1,639.6	1,412.3	1,510.9	1,637.4	1,710.4	1,552.3	1,668.8	6,132.7	6,568.9	6,454.6
Earnings (loss) before income taxes	133.7	209.5	171.7	356.0	84.0	114.8	105.5	321.8	870.9	626.1	479.4
Provision (benefit) for income taxes									12.8	(89.7)	(196.5)
Net earnings (loss)									858.1	715.8	675.9
Net earnings attributable to noncontrolling interests									39.3	47.0	42.4
Net earnings (loss) attributable to controlling interests	142.2	$ 176.6	$ 153.7	$ 346.3	98.5	$ 126.1	$ 110.1	$ 334.1	818.8	668.8	633.5
Net foreign exchange (loss) gain									(2.9)	(6.7)	2.9
Total identifiable assets	22,331.4				19,634.8				22,331.4	19,634.8	16,334.0
Goodwill - net	6,127.0				5,618.5				6,127.0	5,618.5	4,625.6
Amortizable intangible assets - net	2,399.9				2,318.7				2,399.9	2,318.7	1,773.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									5,048.9	5,379.1	5,377.8
Total identifiable assets	11,786.3				10,782.7				11,786.3	10,782.7	9,237.6
Goodwill - net	3,433.0				3,196.9				3,433.0	3,196.9
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									1,034.4	963.4	773.9
Total identifiable assets	6,274.3				5,091.1				6,274.3	5,091.1	3,862.3
Goodwill - net	1,343.4				1,190.7				1,343.4	1,190.7
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									314.5	298.6	290.6
Total identifiable assets	1,462.9				1,307.9				1,462.9	1,307.9	1,143.3
Goodwill - net	473.6				427.0				473.6	427.0
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									249.1	226.0	185.4
Total identifiable assets	1,058.4				916.7				1,058.4	916.7	787.5
Goodwill - net	492.9				454.4				492.9	454.4
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									154.4	160.8	157.5
Total identifiable assets	790.1				718.7				790.1	718.7	709.8
Goodwill - net	232.6				218.1				232.6	218.1
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues									202.3	167.1	148.8
Total identifiable assets	959.4				817.7				959.4	817.7	593.5
Goodwill - net	151.5				131.4				151.5	131.4
Commissions [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									3,591.9	3,320.6	2,920.7
Broker Fees [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									1,957.9	1,911.1	1,756.3
Supplemental Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									221.9	210.5	189.9
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									147.0	135.6	98.0
Investment income [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									75.9	86.9	70.1
Net (Losses) Gains on Divestitures [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									(5.8)	75.3	10.2
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									863.5	1,319.3	1,746.3
Other Net (Losses) Gains [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									(0.4)	(2.9)	0.9
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	6,053.6				5,548.9				6,053.6	5,548.9	4,573.6
Brokerage [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3,399.8				3,163.8				3,399.8	3,163.8
Brokerage [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	1,328.3				1,177.8				1,328.3	1,177.8
Brokerage [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	462.1				416.5				462.1	416.5
Brokerage [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Goodwill - net	492.9				454.4				492.9	454.4
Brokerage [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	221.9				208.0				221.9	208.0
Brokerage [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	148.6				128.4				148.6	128.4
Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									5,167.1	4,901.5	4,246.9
Total revenues									5,167.1	4,901.5	4,246.9
Compensation									2,882.5	2,745.9	2,447.1
Operating									687.2	796.5	673.5
Depreciation									73.5	66.6	60.9
Amortization									411.3	329.1	286.9
Change in estimated acquisition earnout payables									(29.7)	16.9	14.3
Total expenses									4,024.8	3,955.0	3,482.7
Earnings (loss) before income taxes									1,142.3	946.5	764.2
Provision (benefit) for income taxes									276.3	229.2	191.0
Net earnings (loss)									866.0	717.3	573.2
Net earnings attributable to noncontrolling interests									4.9	17.2	10.7
Net earnings (loss) attributable to controlling interests									861.1	700.1	562.5
Net foreign exchange (loss) gain									(2.6)	(1.0)
Total identifiable assets	19,185.3				16,741.9				19,185.3	16,741.9	13,785.1
Goodwill - net	6,053.6				5,548.9				6,053.6	5,548.9	4,573.6
Amortizable intangible assets - net	2,376.3				2,289.9				2,376.3	2,289.9	1,753.7
Brokerage [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									3,369.4	3,234.3	2,840.9
Total identifiable assets	8,897.9				8,132.3				8,897.9	8,132.3	6,865.4
Brokerage [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									993.7	921.8	738.5
Total identifiable assets	6,135.1				4,964.5				6,135.1	4,964.5	3,758.5
Brokerage [Member] | Operating Segments [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									216.1	211.3	195.9
Total identifiable assets	1,373.3				1,217.9				1,373.3	1,217.9	1,096.1
Brokerage [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									243.8	221.4	181.1
Total identifiable assets	1,053.6				913.6				1,053.6	913.6	783.1
Brokerage [Member] | Operating Segments [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									141.8	145.6	141.7
Total identifiable assets	766.0				695.9				766.0	695.9	688.5
Brokerage [Member] | Operating Segments [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues									202.3	167.1	148.8
Total identifiable assets	959.4				817.7				959.4	817.7	593.5
Brokerage [Member] | Commissions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									3,591.9	3,320.6	2,920.7
Brokerage [Member] | Broker Fees [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									1,136.9	1,074.2	958.5
Brokerage [Member] | Supplemental Revenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									221.9	210.5	189.9
Brokerage [Member] | Contingent Revenue [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									147.0	135.6	98.0
Brokerage [Member] | Investment income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									75.2	85.3	69.6
Brokerage [Member] | Net (Losses) Gains on Divestitures [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									(5.8)	75.3	10.2
Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	70.5				66.6				70.5	66.6	49.3
Risk Management [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	33.2				33.1				33.2	33.1
Risk Management [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	15.1				12.9				15.1	12.9
Risk Management [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	11.5				10.5				11.5	10.5
Risk Management [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	10.7				10.1				10.7	10.1
Risk Management [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									821.7	838.5	798.3
Reimbursements									151.7	138.6	141.6
Total revenues									973.4	977.1	939.9
Compensation									517.5	515.7	489.7
Operating									162.6	184.9	174.6
Reimbursements									151.7	138.6	141.6
Depreciation									49.4	46.2	38.7
Amortization									6.0	4.9	4.3
Change in estimated acquisition earnout payables									(3.2)	(1.6)	(4.7)
Total expenses									884.0	888.7	844.2
Earnings (loss) before income taxes									89.4	88.4	95.7
Provision (benefit) for income taxes									22.5	22.2	25.3
Net earnings (loss)									66.9	66.2	70.4
Net earnings (loss) attributable to controlling interests									66.9	66.2	70.4
Net foreign exchange (loss) gain									(0.1)	(0.1)
Total identifiable assets	973.9				898.1				973.9	898.1	748.1
Goodwill - net	70.5				66.6				70.5	66.6	49.3
Amortizable intangible assets - net	23.6				28.8				23.6	28.8	19.3
Risk Management [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									816.4	828.4	789.7
Total identifiable assets	716.2				655.6				716.2	655.6	571.4
Risk Management [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									40.7	41.6	35.4
Total identifiable assets	139.2				126.6				139.2	126.6	103.8
Risk Management [Member] | Operating Segments [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									98.4	87.3	94.7
Total identifiable assets	89.6				90.0				89.6	90.0	47.2
Risk Management [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									5.3	4.6	4.3
Total identifiable assets	4.8				3.1				4.8	3.1	4.4
Risk Management [Member] | Operating Segments [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									12.6	15.2	15.8
Total identifiable assets	24.1				22.8				24.1	22.8	21.3
Risk Management [Member] | Broker Fees [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									821.0	836.9	797.8
Risk Management [Member] | Investment income [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									0.7	1.6	0.5
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2.9				3.0				2.9	3.0	2.7
Corporate [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2.9				3.0				2.9	3.0
Corporate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									863.1	1,316.4	1,747.2
Total revenues									863.1	1,316.4	1,747.2
Compensation									66.5	77.9	89.5
Operating									56.7	87.1	55.6
Cost of revenues from clean coal activities									882.1	1,352.8	1,816.0
Interest									196.4	179.8	138.4
Depreciation									22.2	27.6	28.2
Total expenses									1,223.9	1,725.2	2,127.7
Earnings (loss) before income taxes									(360.8)	(408.8)	(380.5)
Provision (benefit) for income taxes									(286.0)	(341.1)	(412.8)
Net earnings (loss)									(74.8)	(67.7)	32.3
Net earnings attributable to noncontrolling interests									34.4	29.8	31.7
Net earnings (loss) attributable to controlling interests									(109.2)	(97.5)	0.6
Net foreign exchange (loss) gain									(0.2)	(5.6)	2.9
Total identifiable assets	2,172.2				1,994.8				2,172.2	1,994.8	1,800.8
Goodwill - net	2.9				3.0				2.9	3.0	2.7
Corporate [Member] | Operating Segments [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									863.1	1,316.4	1,747.2
Total identifiable assets	$ 2,172.2				$ 1,994.8				2,172.2	1,994.8	1,800.8
Corporate [Member] | Clean Coal Activities [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									863.5	1,319.3	1,746.3
Corporate [Member] | Other Net (Losses) Gains [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									$ (0.4)	$ (2.9)	$ 0.9