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Acquisitions - Summary of Fair Value of Aggregate Consideration Paid (Parenthetical) (Detail) - USD ($) $ in Thousands |  | 9 Months Ended
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Jan. 03, 2019 | Sep. 30, 2019 | Sep. 30, 2018
Business Acquisition [Line Items] |  |  |
Net cash paid |  | $ 237,684 | $ 9,250
SiriusDecisions, Inc [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Cash acquired | $ 7,900 |  |
Net cash paid | 237,700 |  |
Working capital adjustment | $ (1,259) |  |

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