Stockholders' Equity - Summary of Transactions Relating to Common Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock Outstanding Roll Forward [Roll Forward]
Payments for purchases of treasury stock	$ 199,042	$ 260,832	$ 41,272
Common Stock
Common Stock Outstanding Roll Forward [Roll Forward]
Beginning Balance	(163,602,067)	(163,602,067)	(156,234,415)
Issued in connection with the acquisition of CEB			7,367,652
Issuances under stock plans	0	0	0
Purchases for treasury	0	0	0
Ending Balance	(163,602,067)	(163,602,067)	(163,602,067)
Treasury Stock
Common Stock Outstanding Roll Forward [Roll Forward]
Beginning Balance	73,899,977	72,779,205	73,583,172
Issued in connection with the acquisition of CEB			0
Issuances under stock plans	(825,115)	(933,246)	(1,186,150)
Purchases for treasury	1,369,426	2,054,018	382,183
Ending Balance	74,444,288	73,899,977	72,779,205