Revenue from Contracts with Customers - Summary of Consolidated Statement of Earnings Line Items, which Reflect Adoption of New Revenue Recognition Guidance (Detail) - USD ($) $ / shares in Units, $ 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
Revenue, Initial Application Period Cumulative Effect Transition [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]
Cost of revenues from clean coal activities									1,816.0	1,635.9		1,408.6
Operating									903.7	829.1	[1]	776.3	[1]
Reimbursements									141.6	136.0	[1]	132.1	[1]
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 before income taxes									479.4	359.8	[1]	333.6	[1]
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]
Basic net earnings per share									$ 3.47	$ 2.67	[1]	$ 2.23	[1]
Diluted net earnings per share									3.40	2.64	[1]	2.22	[1]
Dividends declared per common share									$ 1.64	$ 1.56	[1]	$ 1.52	[1]
Commissions [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									$ 2,920.7	$ 2,641.0	[1]	$ 2,409.9	[1]
Broker Fees [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									1,756.3	1,591.9	[1]	1,491.7	[1]
Supplemental Revenue Member [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									189.9	158.0	[1]	139.9	[1]
Contingent Revenue [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									98.0	99.5	[1]	97.9	[1]
Investment income [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									70.1	58.7	[1]	53.6	[1]
Gains on Books of Business Sales [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									10.2	3.4	[1]	6.6	[1]
Clean Coal Activities [Member]
Revenue, Initial Application Period Cumulative Effect Transition [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]
Reimbursements [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									141.6	136.0	[1]	132.1	[1]
Product and Service, Other [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues									$ 0.9			(1.3)	[1]
As Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues before reimbursements										6,159.6		5,594.8
Total revenues										6,159.6		5,594.8
Compensation										2,752.3		2,538.9
Operating										852.5		797.7
Interest										124.1		109.8
Depreciation										121.1		103.6
Amortization										264.7		247.2
Change in estimated acquisition earnout payables										30.9		32.1
Total expenses										5,781.5		5,237.9
Earnings before income taxes										378.1		356.9
Benefit for income taxes										(121.1)		(88.1)
Net earnings										499.2		445.0
Net earnings attributable to noncontrolling interests										36.1		30.6
Net earnings attributable to controlling interests										$ 463.1		$ 414.4
Basic net earnings per share										$ 2.57		$ 2.33
Diluted net earnings per share										2.54		2.32
Dividends declared per common share										$ 1.56		$ 1.52
As Previously Reported [Member] | Commissions [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										$ 2,627.1		$ 2,439.1
As Previously Reported [Member] | Broker Fees [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										1,636.8		1,492.8
As Previously Reported [Member] | Supplemental Revenue Member [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										163.7		147.0
As Previously Reported [Member] | Contingent Revenue [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										111.8		107.2
As Previously Reported [Member] | Investment income [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										56.3		53.3
As Previously Reported [Member] | Gains on Books of Business Sales [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										3.4		6.6
As Previously Reported [Member] | Clean Coal Activities [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										1,560.5		1,350.1
Cost of revenues from clean coal activities										1,635.9		1,408.6
As Previously Reported [Member] | Product and Service, Other [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues												(1.3)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues before reimbursements										(46.6)		(46.4)
Total revenues										89.4		85.7
Compensation										(4.9)		(1.7)
Operating										(23.4)		(21.4)
Reimbursements										136.0		132.1
Total expenses										107.7		109.0
Earnings before income taxes										(18.3)		(23.3)
Benefit for income taxes										(36.0)		(8.6)
Net earnings										17.7		(14.7)
Net earnings attributable to noncontrolling interests										(0.5)		2.9
Net earnings attributable to controlling interests										$ 18.2		$ (17.6)
Basic net earnings per share										$ 0.10		$ (0.10)
Diluted net earnings per share										$ 0.10		$ (0.10)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Commissions [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										$ 13.9		$ (29.2)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Broker Fees [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										(44.9)		(1.1)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Supplemental Revenue Member [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										(5.7)		(7.1)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Contingent Revenue [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										(12.3)		(9.3)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Investment income [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										2.4		0.3
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member] | Reimbursements [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues										$ 136.0		$ 132.1

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