Stockholders' Equity (Details) - Summary of Transactions Relating to Common Stock - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock
Common Stock Outstanding Roll Forward [Roll Forward]
Beginning Balance	156,234,415	156,234,415	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	156,234,415	156,234,415
Treasury Stock
Common Stock Outstanding Roll Forward [Roll Forward]
Beginning Balance	73,583,172	73,896,245	68,713,890
Issued in connection with the acquisition of CEB	0
Issuances under stock plans	(1,186,150)	(923,696)	(1,003,746)
Purchases for treasury	382,183	610,623	6,186,101
Ending Balance	72,779,205	73,583,172	73,896,245