Purchase Price Allocation (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Mar. 01, 2016	Dec. 31, 2015	Dec. 31, 2014	Jul. 17, 2014	Jun. 26, 2014
Business Acquisition [Line Items]
Indemnification asset	$ 16.5		$ 19.2
Goodwill	$ 1,023.6		$ 976.3	$ 1,021.1
Gilliland Gold Young (GGY) [Member]
Business Acquisition [Line Items]
Current assets		$ 11.7
Property and equipment, net		2.0
Indemnification asset		1.5
Total intangible assets		34.1
Goodwill		59.4
Liabilities assumed		(22.2)
Net assets acquired		86.5
Gilliland Gold Young (GGY) [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Total intangible assets		3.7
Gilliland Gold Young (GGY) [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Total intangible assets		13.8
Gilliland Gold Young (GGY) [Member] | Software [Member]
Business Acquisition [Line Items]
Total intangible assets		$ 16.6
Web Equity Solutions [Member]
Business Acquisition [Line Items]
Current assets					$ 3.0
Property and equipment, net					2.3
Total intangible assets					54.8
Goodwill					77.6
Liabilities assumed					(7.2)
Net assets acquired					130.5
Web Equity Solutions [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Total intangible assets					0.5
Web Equity Solutions [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Total intangible assets					42.8
Web Equity Solutions [Member] | Software [Member]
Business Acquisition [Line Items]
Total intangible assets					$ 11.5
ICRA [Member]
Business Acquisition [Line Items]
Current assets						$ 25.4
Property and equipment, net						15.1
Total intangible assets						98.9
Goodwill						296.7
Other assets						56.3
Liabilities assumed						(62.7)
Fair value of non-controlling interest assumed						(218.8)
Net assets acquired						210.9
ICRA [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Total intangible assets						46.8
ICRA [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Total intangible assets						33.8
ICRA [Member] | Other Intangibles [Member]
Business Acquisition [Line Items]
Total intangible assets						$ 18.3