Business Combinations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	12 Months Ended			156 Months Ended	0 Months Ended
	Apr. 16, 2014	Dec. 31, 2014 Entity	Dec. 31, 2013 Entity	Dec. 31, 2012 Entity	Dec. 31, 2014 Entity	Oct. 14, 2014	Sep. 30, 2014
Business Acquisition [Line Items]
Cash paid for acquisition of equity interest		$ 1,914.5
Common stock issued for net proceeds	21.85
Net proceeds from common stock issued	911.4	911.4
Accretion of the discount on acquisition		14.5	11.9	9.3
Income related to net adjustments to estimated fair value of liability for earnout obligations		3.0	10.2	5.9
Number of companies acquired		67	79	46	339
Aggregate amount of maximum earnout obligations related to acquisitions		549.8	462.3
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet		205.3	162.7
Goodwill		1,448.3			1,448.3
Expiration lists		895.9			895.9
Non-compete agreements		11.9			11.9
Trade name		9.4			9.4
Total revenues related to acquisitions in the aggregate		4,984.2	3,968.1
Net earnings		316.1	298.4
Brokerage [Member]
Business Acquisition [Line Items]
Goodwill							1,448.3
Expiration lists							895.9
Non-compete agreements							11.9
Trade name							9.4
Oval Group of Companies (OGC) [Member]
Business Acquisition [Line Items]
Cash paid for acquisition of equity interest		338.4
Acquisition agreement date		Apr. 01, 2014
Number of employees at offices of acquired entity		1,000
Number of offices in which acquiree entity operates		24
Goodwill		245.7			245.7
Expiration lists		129.9			129.9
Non-compete agreements		1.0			1.0
Trade name		0.6			0.6
Noraxis Capital Corporation (NCC) [Member]
Business Acquisition [Line Items]
Cash paid for acquisition of equity interest		413.3
Acquisition agreement date		Jul. 02, 2014
Number of employees at offices of acquired entity		650
Percentage of equity on which cash consideration paid		89.00%			89.00%
Goodwill		314.9			314.9
Expiration lists		178.6			178.6
Non-compete agreements		2.6			2.6
Trade name		2.2			2.2
2013 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]
Business Acquisition [Line Items]
Revenue growth rate		2.00%
Discount rate		10.50%
Attrition rate		5.00%
2013 Acquisitions [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]
Business Acquisition [Line Items]
Revenue growth rate		3.00%
Discount rate		15.00%
Attrition rate		11.50%
Business Acquisition [Member]
Business Acquisition [Line Items]
Annualized revenue of business acquisitions		761.2
Total revenues related to acquisitions in the aggregate		413.0
Net earnings		37.8
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists		895.9			895.9
Non-compete agreements		11.9			11.9
Trade name		9.4			9.4
Deferred tax liability		173.9			173.9
Goodwill related to nondeductible amortizable intangible assets		173.9
Business Acquisition [Member] | Brokerage [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes		679.3			679.3
Business Acquisition [Member] | Brokerage [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes		9.9			9.9
Business Acquisition [Member] | Brokerage [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes		7.1			7.1
Crombie Lockwood Oamps [Member]
Business Acquisition [Line Items]
Cash paid for acquisition of equity interest		952.0
Acquisition agreement date		Jun. 16, 2014
Number of employees at offices of acquired entity		1,700
Number of offices in which acquiree entity operates		50
Payment for excess of net current assets						$ 35.3
2014 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]
Business Acquisition [Line Items]
Revenue growth rate		4.00%
Discount rate		8.50%
2014 Acquisitions [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]
Business Acquisition [Line Items]
Revenue growth rate		12.00%
Discount rate		9.50%