Stockholders' Equity - Summary of Transactions Relating to Common Stock (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock
Common Stock Outstanding Roll Forward [Roll Forward]
Beginning Balance	163,602,067	156,234,415	156,234,415
Issuances under stock plans	0	0	0
Issued in connection with the acquisition of CEB		7,367,652
Purchases for treasury	0	0	0
Ending Balance	163,602,067	163,602,067	156,234,415
Treasury Stock
Common Stock Outstanding Roll Forward [Roll Forward]
Beginning Balance	72,779,205	73,583,172	73,896,245
Issuances under stock plans	(933,246)	(1,186,150)	(923,696)
Issued in connection with the acquisition of CEB		0
Purchases for treasury	2,054,018	382,183	610,623
Ending Balance	73,899,977	72,779,205	73,583,172