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Acquisitions - Schedule of Unaudited Pro Forma Financial Information (Detail) - SiriusDecisions, Inc [Member] - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Business Acquisition [Line Items] |  |  |  |
Pro forma total revenue | $ 110,224 | $ 101,848 | $ 347,751 | $ 318,094
Pro forma net loss | $ (1,092) | $ (2,511) | $ (5,359) | $ (9,951)

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