Acquisitions (Details) enterprise in Thousands			1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 05, 2017 USD ($) employee country	Mar. 09, 2017 USD ($) employee	Apr. 05, 2017 USD ($) employee country	Sep. 30, 2017 USD ($) employee country enterprise	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) employee country acquisition enterprise	Sep. 30, 2016 USD ($)
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%		100.00%
Consideration transferred, aggregate purchase price			$ 3,653,028,000
Consideration transferred, cash paid			$ 2,821,903,000
Entity number of employees | employee				14,000		14,000
Number of corporations served | enterprise				11		11
Number of countries in which entity operates | country				100		100
Net cash paid for acquisition						$ 2,630,000,000
Number of acquisitions | acquisition						2
Acquisition and integration charges				$ 30,500,000	$ 16,557,000	$ 142,104,000	$ 32,958,000
CEB
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%		100.00%
Consideration transferred, aggregate purchase price	$ 3,518,829,000
Consideration transferred, cash paid	2,687,704,000			$ 12,500,000
Consideration transferred, fair value of common shares issued	$ 818,700,000		$ 818,700,000
Entity number of employees | employee	4,900		4,900
Number of countries in which entity operates | country	70		70
Proceeds from issuance of debt	$ 2,780,000,000
Goodwill, expected to be deductible for tax purposes	$ 0		$ 0
L2, Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Consideration transferred, aggregate purchase price		$ 134,199,000
Consideration transferred, cash paid		$ 134,199,000
Entity number of employees | employee		150
Contingent consideration arrangements, maximum value		$ 20,800,000
Contingent consideration arrangements, maximum value, period of expense recognition		3 years
Goodwill, expected to be deductible for tax purposes		$ 0