Acquisitions - (Details)			1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 05, 2017 USD ($) employee country enterprise	Mar. 09, 2017 USD ($) employee	Apr. 05, 2017 USD ($) employee country acquisition enterprise	Jun. 30, 2017 USD ($) employee country enterprise	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) employee country acquisition enterprise	Jun. 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				13,000		13,000
Number of corporations served | enterprise				11,000		11,000
Number of countries in which entity operates | country				100		100
Net cash paid for acquisition			$ 2,620,000,000
Number of acquisitions | acquisition			2			2
Goodwill, expected to be deductible for tax purposes	$ 0		$ 0
Acquisition and integration charges				$ 98,332,000	$ 8,033,000	$ 111,604,000	$ 16,401,000
Acquisition and integration charges, exit costs						20,800,000
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
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 corporations served | enterprise	10,000		10,000
Number of countries in which entity operates | country	70		70
Additional cash to be paid	$ 12,465,000
Proceeds from issuance of debt	2,780,000,000
Goodwill, expected to be deductible for tax purposes	$ 0		$ 0
Additional revenue recognized resulting from acquisition				$ 152,600,000		$ 152,600,000
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
Additional cash to be paid		$ 20,800,000
Contingent consideration arrangements, maximum value, period of expense recognition		3 years
Goodwill, expected to be deductible for tax purposes		$ 0