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Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues before reimbursements										$ 6,792.4	$ 6,113.0	[1]	$ 5,548.4	[1]
Total revenues										6,934.0	6,249.0	[1]	5,680.5	[1]
Compensation										3,026.3	2,747.4	[1]	2,537.2	[1]
Operating										903.7	829.1	[1]	776.3	[1]
Reimbursements										141.6	136.0	[1]	132.1	[1]
Cost of revenues from clean coal activities										1,816.0	1,635.9		1,408.6
Interest										138.4	124.1	[1]	109.8	[1]
Depreciation										127.8	121.1	[1]	103.6	[1]
Amortization										291.2	264.7	[1]	247.2	[1]
Change in estimated acquisition earnout payables										9.6	30.9	[1]	32.1	[1]
Total expenses										6,454.6	5,889.2	[1]	5,346.9	[1]
Earnings (loss) before income taxes										479.4	359.8	[1]	333.6	[1]
Provision (benefit) for income taxes										(196.5)	(157.1)	[1]	(96.7)	[1]
Net earnings										675.9	516.9	[1]	430.3	[1]
Net earnings attributable to noncontrolling interests										42.4	35.6	[1]	33.5	[1]
Net earnings attributable to controlling interests	$ 117.3	$ 127.6	$ 114.9	$ 273.7	$ 71.5		$ 111.0	$ 70.0	$ 228.8	633.5	481.3	[1]	396.8	[1]
Net foreign exchange gain										2.9	(3.9)		5.3
Total revenues										6,934.0	6,249.0	[1]	5,680.5	[1]
Total identifiable assets	16,334.0				14,909.7	[1]				16,334.0	14,909.7	[1]	13,528.2
Goodwill - net	4,625.6				4,164.8	[1]				4,625.6	4,164.8	[1]	3,753.2
Amortizable intangible assets - net	1,773.0				1,644.6	[1]				1,773.0	1,644.6	[1]	1,627.3
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues										5,377.8	4,839.3		4,389.2
Total revenues										5,377.8	4,839.3		4,389.2
Total identifiable assets	9,237.6				8,230.2					9,237.6	8,230.2		7,638.8
Goodwill - net	2,744.9				2,306.7					2,744.9	2,306.7
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues										773.9	709.9		688.3
Total revenues										773.9	709.9		688.3
Total identifiable assets	3,862.3				3,587.5					3,862.3	3,587.5		3,234.7
Goodwill - net	762.9				745.7					762.9	745.7
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues										290.6	269.3		243.0
Total revenues										290.6	269.3		243.0
Total identifiable assets	1,143.3				1,151.8					1,143.3	1,151.8		988.7
Goodwill - net	406.6				416.6					406.6	416.6
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues										185.4	153.6		136.5
Total revenues										185.4	153.6		136.5
Total identifiable assets	787.5				750.1					787.5	750.1		587.0
Goodwill - net	378.6				374.0					378.6	374.0
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues										157.5	149.7		125.5
Total revenues										157.5	149.7		125.5
Total identifiable assets	709.8				728.6					709.8	728.6		676.4
Goodwill - net	219.8				218.9					219.8	218.9
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues										148.8	127.2		98.0
Total revenues										148.8	127.2		98.0
Total identifiable assets	593.5				461.5					593.5	461.5		402.6
Goodwill - net	112.8				102.9					112.8	102.9
Brokerage [Member]
Segment Reporting Information [Line Items]
Goodwill - net	4,573.6				4,119.2					4,573.6	4,119.2		3,722.3
Brokerage [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2,715.3				2,280.9					2,715.3	2,280.9
Brokerage [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	753.7				738.5					753.7	738.5
Brokerage [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	406.3				416.6					406.3	416.6
Brokerage [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Goodwill - net	378.6				374.0					378.6	374.0
Brokerage [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	209.6				209.3					209.6	209.3
Brokerage [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	110.1				99.9					110.1	99.9
Risk Management [Member]
Segment Reporting Information [Line Items]
Goodwill - net	49.3				42.6					49.3	42.6		28.1
Risk Management [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Goodwill - net	29.6				25.8					29.6	25.8
Risk Management [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Goodwill - net	9.2				7.2					9.2	7.2
Risk Management [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Goodwill - net	0.3									0.3
Risk Management [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Goodwill - net	10.2				9.6					10.2	9.6
Corporate [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2.7				3.0					2.7	3.0		2.8
Corporate [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Goodwill - net	2.7				3.0					2.7	3.0
Commissions [Member]
Segment Reporting Information [Line Items]
Total revenues										2,920.7	2,641.0	[1]	2,409.9	[1]
Total revenues										2,920.7	2,641.0	[1]	2,409.9	[1]
Broker Fees [Member]
Segment Reporting Information [Line Items]
Total revenues										1,756.3	1,591.9	[1]	1,491.7	[1]
Total revenues										1,756.3	1,591.9	[1]	1,491.7	[1]
Supplemental Revenue Member [Member]
Segment Reporting Information [Line Items]
Total revenues										189.9	158.0	[1]	139.9	[1]
Total revenues										189.9	158.0	[1]	139.9	[1]
Contingent Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues										98.0	99.5	[1]	97.9	[1]
Total revenues										98.0	99.5	[1]	97.9	[1]
Investment income [Member]
Segment Reporting Information [Line Items]
Total revenues										70.1	58.7	[1]	53.6	[1]
Total revenues										70.1	58.7	[1]	53.6	[1]
Gains on Books of Business Sales [Member]
Segment Reporting Information [Line Items]
Total revenues										10.2	3.4	[1]	6.6	[1]
Total revenues										10.2	3.4	[1]	6.6	[1]
Clean Coal Activities [Member]
Segment Reporting Information [Line Items]
Total revenues										1,746.3	1,560.5	[1]	1,350.1	[1]
Cost of revenues from clean coal activities										1,816.0	1,635.9	[1]	1,408.6	[1]
Total revenues										1,746.3	1,560.5	[1]	1,350.1	[1]
Product and Service, Other [Member]
Segment Reporting Information [Line Items]
Total revenues										0.9			(1.3)	[1]
Total revenues										0.9			(1.3)	[1]
Reimbursements [Member]
Segment Reporting Information [Line Items]
Total revenues										141.6	136.0	[1]	132.1	[1]
Total revenues										141.6	136.0	[1]	132.1	[1]
Operating Segments [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements										4,246.9	3,815.1		3,501.6
Total revenues										4,246.9	3,815.1		3,501.6
Compensation										2,447.1	2,212.3		2,040.2
Operating										673.5	614.0		598.2
Depreciation										60.9	61.8		57.2
Amortization										286.9	261.8		244.7
Change in estimated acquisition earnout payables										14.3	29.3		32.1
Total expenses										3,482.7	3,179.2		2,972.4
Earnings (loss) before income taxes										764.2	635.9		529.2
Provision (benefit) for income taxes										191.0	221.2		186.6
Net earnings										573.2	414.7		342.6
Net earnings attributable to noncontrolling interests										10.7	7.6		6.5
Net earnings attributable to controlling interests										562.5	407.1		336.1
Net foreign exchange gain											(2.0)		5.2
Total revenues										4,246.9	3,815.1		3,501.6
Total identifiable assets	13,785.1				12,404.3					13,785.1	12,404.3		11,308.6
Goodwill - net	4,573.7				4,119.2					4,573.7	4,119.2		3,722.3
Amortizable intangible assets - net	1,753.7				1,630.6					1,753.7	1,630.6		1,613.6
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues										2,840.9	2,533.7		2,319.3
Total revenues										2,840.9	2,533.7		2,319.3
Total identifiable assets	6,865.4				5,890.5					6,865.4	5,890.5		5,545.1
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues										738.5	679.3		661.5
Total revenues										738.5	679.3		661.5
Total identifiable assets	3,758.5				3,496.2					3,758.5	3,496.2		3,172.9
Operating Segments [Member] | Brokerage [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues										195.9	191.1		170.0
Total revenues										195.9	191.1		170.0
Total identifiable assets	1,096.1				1,102.9					1,096.1	1,102.9		931.8
Operating Segments [Member] | Brokerage [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues										181.1	149.4		132.4
Total revenues										181.1	149.4		132.4
Total identifiable assets	783.1				743.3					783.1	743.3		584.2
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues										141.7	134.4		120.4
Total revenues										141.7	134.4		120.4
Total identifiable assets	688.5				709.9					688.5	709.9		672.0
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues										148.8	127.2		98.0
Total revenues										148.8	127.2		98.0
Total identifiable assets	593.5				461.5					593.5	461.5		402.6
Operating Segments [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements										798.3	737.4		698.0
Total revenues										939.9	873.4		830.1
Compensation										489.7	446.9		424.4
Operating										174.6	164.8		152.7
Reimbursements										141.6	136.0		132.1
Depreciation										38.7	31.1		27.2
Amortization										4.3	2.9		2.5
Change in estimated acquisition earnout payables										(4.7)	1.6
Total expenses										844.2	783.3		738.9
Earnings (loss) before income taxes										95.7	90.1		91.2
Provision (benefit) for income taxes										25.3	34.4		34.5
Net earnings										70.4	55.7		56.7
Net earnings attributable to controlling interests										70.4	55.7		56.7
Net foreign exchange gain											(0.1)
Total revenues										939.9	873.4		830.1
Total identifiable assets	748.1				738.6					748.1	738.6		670.9
Goodwill - net	49.2				42.6					49.2	42.6		28.1
Amortizable intangible assets - net	19.3				14.0					19.3	14.0		13.7
Operating Segments [Member] | Risk Management [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues										789.7	745.1		721.1
Total revenues										789.7	745.1		721.1
Total identifiable assets	571.4				572.9					571.4	572.9		545.0
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues										35.4	30.6		26.8
Total revenues										35.4	30.6		26.8
Total identifiable assets	103.8				91.3					103.8	91.3		61.8
Operating Segments [Member] | Risk Management [Member] | Australia [Member]
Segment Reporting Information [Line Items]
Total revenues										94.7	78.2		73.0
Total revenues										94.7	78.2		73.0
Total identifiable assets	47.2				48.9					47.2	48.9		56.9
Operating Segments [Member] | Risk Management [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total revenues										4.3	4.2		4.1
Total revenues										4.3	4.2		4.1
Total identifiable assets	4.4				6.8					4.4	6.8		2.8
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues										15.8	15.3		5.1
Total revenues										15.8	15.3		5.1
Total identifiable assets	21.3				18.7					21.3	18.7		4.4
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Revenues before reimbursements										1,747.2	1,560.5		1,348.8
Total revenues										1,747.2	1,560.5		1,348.8
Compensation										89.5	88.2		72.6
Operating										55.6	50.3		25.4
Cost of revenues from clean coal activities										1,816.0	1,635.9		1,408.6
Interest										138.4	124.1		109.8
Depreciation										28.2	28.2		19.2
Total expenses										2,127.7	1,926.7		1,635.6
Earnings (loss) before income taxes										(380.5)	(366.2)		(286.8)
Provision (benefit) for income taxes										(412.8)	(412.7)		(317.8)
Net earnings										32.3	46.5		31.0
Net earnings attributable to noncontrolling interests										31.7	28.0		27.0
Net earnings attributable to controlling interests										0.6	18.5		4.0
Net foreign exchange gain										2.9	(1.8)		0.1
Total revenues										1,747.2	1,560.5		1,348.8
Total identifiable assets	1,800.8				1,766.8					1,800.8	1,766.8		1,548.7
Goodwill - net	2.7				3.0					2.7	3.0		2.8
Operating Segments [Member] | Corporate [Member] | Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues										1,747.2	1,560.5		1,348.8
Total revenues										1,747.2	1,560.5		1,348.8
Total identifiable assets	$ 1,800.8				$ 1,766.8					1,800.8	1,766.8		1,548.7
Operating Segments [Member] | Commissions [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										2,920.7	2,641.0		2,409.9
Total revenues										2,920.7	2,641.0		2,409.9
Operating Segments [Member] | Broker Fees [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										958.5	855.1		794.7
Total revenues										958.5	855.1		794.7
Operating Segments [Member] | Broker Fees [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues										797.8	736.8		697.0
Total revenues										797.8	736.8		697.0
Operating Segments [Member] | Supplemental Revenue Member [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										189.9	158.0		139.9
Total revenues										189.9	158.0		139.9
Operating Segments [Member] | Contingent Revenue [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										98.0	99.5		97.9
Total revenues										98.0	99.5		97.9
Operating Segments [Member] | Investment income [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										69.6	58.1		52.6
Total revenues										69.6	58.1		52.6
Operating Segments [Member] | Investment income [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues										0.5	0.6		1.0
Total revenues										0.5	0.6		1.0
Operating Segments [Member] | Gains on Books of Business Sales [Member] | Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues										10.2	3.4		6.6
Total revenues										10.2	3.4		6.6
Operating Segments [Member] | Clean Coal Activities [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues										1,746.3	1,560.5		1,350.1
Total revenues										1,746.3	1,560.5		1,350.1
Operating Segments [Member] | Product and Service, Other [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Total revenues										0.9			(1.3)
Total revenues										0.9			(1.3)
Operating Segments [Member] | Reimbursements [Member] | Risk Management [Member]
Segment Reporting Information [Line Items]
Total revenues										141.6	136.0		132.1
Total revenues										$ 141.6	$ 136.0		$ 132.1
[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.

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