Business and Significant Accounting Policies (Details) - Changes to the Carrying Amount of Goodwill by Reporting Unit - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning Balance	$ 738,453	$ 715,359
Additions due to acquisition	2,457,791	34,308
Foreign currency translation impact	4,044	(11,214)
Ending Balance	2,987,294	738,453
Not Discontinued Operations
Goodwill [Roll Forward]
Reclassified as held-for-sale	(212,994)
Research
Goodwill [Roll Forward]
Beginning Balance	595,450	575,292
Additions due to acquisition	2,042,514	28,465
Foreign currency translation impact	(18,287)	(8,307)
Ending Balance	2,619,677	595,450
Research | Not Discontinued Operations
Goodwill [Roll Forward]
Reclassified as held-for-sale	0
Consulting
Goodwill [Roll Forward]
Beginning Balance	96,480	98,412
Additions due to acquisition	0	0
Foreign currency translation impact	1,318	(1,932)
Ending Balance	97,798	96,480
Consulting | Not Discontinued Operations
Goodwill [Roll Forward]
Reclassified as held-for-sale	0
Events
Goodwill [Roll Forward]
Beginning Balance	46,523	41,655
Additions due to acquisition	140,914	5,843
Foreign currency translation impact	483	(975)
Ending Balance	187,920	46,523
Events | Not Discontinued Operations
Goodwill [Roll Forward]
Reclassified as held-for-sale	0
Talent Assessment & Other
Goodwill [Roll Forward]
Beginning Balance	0	0
Additions due to acquisition	274,363	0
Foreign currency translation impact	20,530	0
Ending Balance	81,899	$ 0
Talent Assessment & Other | Not Discontinued Operations
Goodwill [Roll Forward]
Reclassified as held-for-sale	$ (212,994)