Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended	6 Months Ended
	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill [Roll Forward]
Balance, December 31, 2017		$ 2,987,294,000
Divestiture		(20,642,000)
Foreign currency translation impact and other		7,861,000
Balance at June 30, 2018		2,974,513,000
Goodwill, accumulated impairment losses			$ 0
Research
Goodwill [Roll Forward]
Balance, December 31, 2017		2,619,677,000
Divestiture		0
Foreign currency translation impact and other		568,000
Balance at June 30, 2018		2,620,245,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2017		97,798,000
Divestiture		0
Foreign currency translation impact and other		(335,000)
Balance at June 30, 2018		97,463,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2017		187,920,000
Divestiture		0
Foreign currency translation impact and other		(245,000)
Balance at June 30, 2018		187,675,000
Talent Assessment & Other
Goodwill [Roll Forward]
Balance, December 31, 2017		81,899,000
Divestiture		(20,642,000)
Foreign currency translation impact and other		7,873,000
Balance at June 30, 2018		$ 69,130,000