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Fair Value Measurements - Summary of Reconciliation of Recurring Fair Value Measurements (Detail) - Deferred Consideration Obligation [Member] - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Beginning balance | $ 162,848 | $ 172,746
Net realized losses/(gains) | 2,880 | 5,595
Net unrealized losses/(gains) | (7,732) | (17,630)
Settlements | (1,941) | (4,656)
Ending balance | $ 156,055 | $ 156,055

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