Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues before reimbursements									$ 7,056.4	$ 6,792.4	$ 6,113.0
Total revenues									7,195.0	6,934.0	6,249.0
Compensation									3,339.5	3,026.3	2,747.4
Operating									1,068.5	903.7	829.1
Reimbursements									138.6	141.6	136.0
Cost of revenues from clean coal activities									1,352.8	1,816.0	1,635.9
Interest									179.8	138.4	124.1
Depreciation									140.4	127.8	121.1
Amortization									334.0	291.2	264.7
Change in estimated acquisition earnout payables									15.3	9.6	30.9
Total expenses									6,568.9	6,454.6	5,889.2
Earnings (loss) before income taxes									626.1	479.4	359.8
Provision (benefit) for income taxes									(89.7)	(196.5)	(157.1)
Net earnings (loss)									715.8	675.9	516.9
Net earnings attributable to noncontrolling interests									47.0	42.4	35.6
Net earnings (loss) attributable to controlling interests	$ 98.5	$ 126.1	$ 110.1	$ 334.1	$ 117.3	$ 127.6	$ 114.9	$ 273.7	668.8	633.5	481.3	[1]
Net foreign exchange gain									(6.7)	2.9	(3.9)
Total revenues									7,195.0	6,934.0	6,249.0
Total identifiable assets	19,634.8				16,334.0				19,634.8	16,334.0	14,909.7
Goodwill - net	5,618.5				4,625.6				5,618.5	4,625.6	4,164.8
Amortizable intangible assets - net	2,318.7				1,773.0				2,318.7	1,773.0	1,644.6
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									5,379.1	5,377.8	4,839.3
Total revenues									5,379.1	5,377.8	4,839.3
Total identifiable assets	10,782.7				9,237.6				10,782.7	9,237.6	8,230.2
Goodwill - net	3,196.9				2,744.9				3,196.9	2,744.9
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									963.4	773.9	709.9
Total revenues									963.4	773.9	709.9
Total identifiable assets	5,091.1				3,862.3				5,091.1	3,862.3	3,587.5
Goodwill - net	1,190.7				762.9				1,190.7	762.9
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									298.6	290.6	269.3
Total revenues									298.6	290.6	269.3
Total identifiable assets	1,307.9				1,143.3				1,307.9	1,143.3	1,151.8
Goodwill - net	427.0				406.6				427.0	406.6
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									226.0	185.4	153.6
Total revenues									226.0	185.4	153.6
Total identifiable assets	916.7				787.5				916.7	787.5	750.1
Goodwill - net	454.4				378.6				454.4	378.6
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									160.8	157.5	149.7
Total revenues									160.8	157.5	149.7
Total identifiable assets	718.7				709.8				718.7	709.8	728.6
Goodwill - net	218.1				219.8				218.1	219.8
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues									167.1	148.8	127.2
Total revenues									167.1	148.8	127.2
Total identifiable assets	817.7				593.5				817.7	593.5	461.5
Goodwill - net	131.4				112.8				131.4	112.8
Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									4,901.5
Total revenues									4,901.5
Goodwill - net	5,548.9				4,573.6				5,548.9	4,573.6	4,119.2
Brokerage [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3,163.8				2,715.3				3,163.8	2,715.3
Brokerage [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	1,177.8				753.7				1,177.8	753.7
Brokerage [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	416.5				406.3				416.5	406.3
Brokerage [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Goodwill - net	454.4				378.6				454.4	378.6
Brokerage [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	208.0				209.6				208.0	209.6
Brokerage [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	128.4				110.1				128.4	110.1
Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									977.1
Total revenues									977.1
Goodwill - net	66.6				49.3				66.6	49.3	42.6
Risk Management [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	33.1				29.6				33.1	29.6
Risk Management [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	12.9				9.2				12.9	9.2
Risk Management [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	10.5				0.3				10.5	0.3
Risk Management [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	10.1				10.2				10.1	10.2
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									1,316.4
Total revenues									1,316.4
Goodwill - net	3.0				2.7				3.0	2.7	3.0
Corporate [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	3.0				2.7				3.0	2.7
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues									3,320.6	2,920.7	2,641.0
Total revenues									3,320.6	2,920.7	2,641.0
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues									1,911.1	1,756.3	1,591.9
Total revenues									1,911.1	1,756.3	1,591.9
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues									210.5	189.9	158.0
Total revenues									210.5	189.9	158.0
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues									135.6	98.0	99.5
Total revenues									135.6	98.0	99.5
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues									86.9	70.1	58.7
Total revenues									86.9	70.1	58.7
Gains on Books of Business Sales [Member]
Segment Reporting Information [Line Items]
Total revenues									75.3	10.2	3.4
Total revenues									75.3	10.2	3.4
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues									1,319.3	1,746.3	1,560.5
Total revenues									1,319.3	1,746.3	1,560.5
Product and Service, Other [Member]
Segment Reporting Information [Line Items]
Total revenues									(2.9)	0.9
Total revenues									(2.9)	0.9
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues									138.6	141.6	136.0
Total revenues									138.6	141.6	136.0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues											6,249.0
Total revenues											6,249.0
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									4,901.5	4,246.9	3,815.1
Total revenues									4,901.5	4,246.9	3,815.1
Compensation									2,745.9	2,447.1	2,212.3
Operating									796.5	673.5	614.0
Depreciation									66.6	60.9	61.8
Amortization									329.1	286.9	261.8
Change in estimated acquisition earnout payables									16.9	14.3	29.3
Total expenses									3,955.0	3,482.7	3,179.2
Earnings (loss) before income taxes									946.5	764.2	635.9
Provision (benefit) for income taxes									229.2	191.0	221.2
Net earnings (loss)									717.3	573.2	414.7
Net earnings attributable to noncontrolling interests									17.2	10.7	7.6
Net earnings (loss) attributable to controlling interests									700.1	562.5	407.1
Net foreign exchange gain									(1.0)		(2.0)
Total revenues									4,901.5	4,246.9	3,815.1
Total identifiable assets	16,741.9				13,785.1				16,741.9	13,785.1	12,404.3
Goodwill - net	5,548.9				4,573.7				5,548.9	4,573.7	4,119.2
Amortizable intangible assets - net	2,289.9				1,753.7				2,289.9	1,753.7	1,630.6
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									3,234.3	2,840.9	2,533.7
Total revenues									3,234.3	2,840.9	2,533.7
Total identifiable assets	8,132.3				6,865.4				8,132.3	6,865.4	5,890.5
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									921.8	738.5	679.3
Total revenues									921.8	738.5	679.3
Total identifiable assets	4,964.5				3,758.5				4,964.5	3,758.5	3,496.2
Operating Segments [Member] | Brokerage [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									211.3	195.9	191.1
Total revenues									211.3	195.9	191.1
Total identifiable assets	1,217.9				1,096.1				1,217.9	1,096.1	1,102.9
Operating Segments [Member] | Brokerage [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									221.4	181.1	149.4
Total revenues									221.4	181.1	149.4
Total identifiable assets	913.6				783.1				913.6	783.1	743.3
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									145.6	141.7	134.4
Total revenues									145.6	141.7	134.4
Total identifiable assets	695.9				688.5				695.9	688.5	709.9
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues									167.1	148.8	127.2
Total revenues									167.1	148.8	127.2
Total identifiable assets	817.7				593.5				817.7	593.5	461.5
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									838.5	798.3	737.4
Total revenues									977.1	939.9	873.4
Compensation									515.7	489.7	446.9
Operating									184.9	174.6	164.8
Reimbursements									138.6	141.6	136.0
Depreciation									46.2	38.7	31.1
Amortization									4.9	4.3	2.9
Change in estimated acquisition earnout payables									(1.6)	(4.7)	1.6
Total expenses									888.7	844.2	783.3
Earnings (loss) before income taxes									88.4	95.7	90.1
Provision (benefit) for income taxes									22.2	25.3	34.4
Net earnings (loss)									66.2	70.4	55.7
Net earnings (loss) attributable to controlling interests									66.2	70.4	55.7
Net foreign exchange gain									(0.1)		(0.1)
Total revenues									977.1	939.9	873.4
Total identifiable assets	898.1				748.1				898.1	748.1	738.6
Goodwill - net	66.6				49.2				66.6	49.2	42.6
Amortizable intangible assets - net	28.8				19.3				28.8	19.3	14.0
Operating Segments [Member] | Risk Management [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									828.4	789.7	745.1
Total revenues									828.4	789.7	745.1
Total identifiable assets	655.6				571.4				655.6	571.4	572.9
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues									41.6	35.4	30.6
Total revenues									41.6	35.4	30.6
Total identifiable assets	126.6				103.8				126.6	103.8	91.3
Operating Segments [Member] | Risk Management [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues									87.3	94.7	78.2
Total revenues									87.3	94.7	78.2
Total identifiable assets	90.0				47.2				90.0	47.2	48.9
Operating Segments [Member] | Risk Management [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues									4.6	4.3	4.2
Total revenues									4.6	4.3	4.2
Total identifiable assets	3.1				4.4				3.1	4.4	6.8
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues									15.2	15.8	15.3
Total revenues									15.2	15.8	15.3
Total identifiable assets	22.8				21.3				22.8	21.3	18.7
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements									1,316.4	1,747.2	1,560.5
Total revenues									1,316.4	1,747.2	1,560.5
Compensation									77.9	89.5	88.2
Operating									87.1	55.6	50.3
Cost of revenues from clean coal activities									1,352.8	1,816.0	1,635.9
Interest									179.8	138.4	124.1
Depreciation									27.6	28.2	28.2
Total expenses									1,725.2	2,127.7	1,926.7
Earnings (loss) before income taxes									(408.8)	(380.5)	(366.2)
Provision (benefit) for income taxes									(341.1)	(412.8)	(412.7)
Net earnings (loss)									(67.7)	32.3	46.5
Net earnings attributable to noncontrolling interests									29.8	31.7	28.0
Net earnings (loss) attributable to controlling interests									(97.5)	0.6	18.5
Net foreign exchange gain									(5.6)	2.9	(1.8)
Total revenues									1,316.4	1,747.2	1,560.5
Total identifiable assets	1,994.8				1,800.8				1,994.8	1,800.8	1,766.8
Goodwill - net	3.0				2.7				3.0	2.7	3.0
Operating Segments [Member] | Corporate [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues									1,316.4	1,747.2	1,560.5
Total revenues									1,316.4	1,747.2	1,560.5
Total identifiable assets	$ 1,994.8				$ 1,800.8				1,994.8	1,800.8	1,766.8
Operating Segments [Member] | Commissions [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									3,320.6	2,920.7	2,641.0
Total revenues									3,320.6	2,920.7	2,641.0
Operating Segments [Member] | Broker Fees [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									1,074.2	958.5	855.1
Total revenues									1,074.2	958.5	855.1
Operating Segments [Member] | Broker Fees [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									836.9	797.8	736.8
Total revenues									836.9	797.8	736.8
Operating Segments [Member] | Supplemental Revenue Member [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									210.5	189.9	158.0
Total revenues									210.5	189.9	158.0
Operating Segments [Member] | Contingent Revenue [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									135.6	98.0	99.5
Total revenues									135.6	98.0	99.5
Operating Segments [Member] | Investment income [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									85.3	69.6	58.1
Total revenues									85.3	69.6	58.1
Operating Segments [Member] | Investment income [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									1.6	0.5	0.6
Total revenues									1.6	0.5	0.6
Operating Segments [Member] | Gains on Books of Business Sales [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues									75.3	10.2	3.4
Total revenues									75.3	10.2	3.4
Operating Segments [Member] | Clean Coal Activities [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									1,319.3	1,746.3	1,560.5
Total revenues									1,319.3	1,746.3	1,560.5
Operating Segments [Member] | Product and Service, Other [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues									(2.9)	0.9
Total revenues									(2.9)	0.9
Operating Segments [Member] | Reimbursements [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues									138.6	141.6	136.0
Total revenues									$ 138.6	$ 141.6	$ 136.0

[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.