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Operating Segment and Enterprise Wide Reporting - Schedule of Information about Reportable Segments (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Segment Reporting Information [Line Items]                      
Research services revenues                 $ 215,216 $ 210,268 $ 207,517
Advisory services and events revenues                 110,879 103,458 104,545
Total revenues $ 83,446 $ 77,427 $ 87,821 $ 77,401 $ 80,975 $ 74,753 $ 82,811 $ 75,187 326,095 313,726 312,062
Segment expenses                 119,307 117,445 120,009
Contribution margin (loss)                 206,788 196,281 192,053
Selling, marketing, administrative and other expenses                 (174,157) (172,130) (169,852)
Amortization of intangible assets                 (831) (891) (2,171)
Reorganization costs                 (1,026) (4,433) (1,817)
Other income and losses on investments                 (65) 493 176
Income before income taxes                 30,709 19,320 18,389
Product Segment [Member]                      
Segment Reporting Information [Line Items]                      
Research services revenues                 215,216 210,268 207,517
Advisory services and events revenues                 20,374 17,512 20,759
Total revenues                 235,590 227,780 228,276
Segment expenses                 41,528 38,615 39,466
Contribution margin (loss)                 194,062 189,165 188,810
Research Segment [Member]                      
Segment Reporting Information [Line Items]                      
Advisory services and events revenues                 45,601 42,997 48,658
Total revenues                 45,601 42,997 48,658
Segment expenses                 51,132 51,510 53,307
Contribution margin (loss)                 (5,531) (8,513) (4,649)
Project Consulting [Member]                      
Segment Reporting Information [Line Items]                      
Advisory services and events revenues                 44,904 42,949 35,128
Total revenues                 44,904 42,949 35,128
Segment expenses                 26,647 27,320 27,236
Contribution margin (loss)                 $ 18,257 $ 15,629 $ 7,892