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Schedule of Changes in Fair Value of Level 3 Contingent Consideration (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Balance at beginning of period |  | $ 38,332 |  | $ 37,436 |
Payments |  | (1,050) | [1] | (15,000) | [2]
Ending Balance |  | 45,102 |  | 38,332 |
Avionics Interface Technologies, LLC |  |  |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Fair value adjustment | [3] |  |  | 550 |
Universal Robots |  |  |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |
Fair value adjustment |  | $ 7,820 | [4] | $ 15,346 | [3]
[1] | During the year ended December 31, 2017, Teradyne paid $1.1 million of contingent consideration for the earn-out in connection with the acquisition of AIT.
[2] | During the year ended December 31, 2016, based on Universal Robots' calendar year 2015 EBITDA results, Teradyne paid $15 million or 100% of the eligible EBITDA contingent consideration amount.
[3] | During the year ended December 31, 2016, the fair value of contingent consideration for the earn-out in connection with the acquisition of Universal Robots was increased by $15.3 million primarily due to an increase in forecasted revenues and a decrease in the discount rate. During the year ended 2016, the fair value of contingent consideration for the earn-out in connection with the acquisition of Avionics Interface Technologies, LLC ("AIT") was increased by $0.6 million due to an increase in forecasted revenues.
[4] | During the year ended December 31, 2017, the fair value of contingent consideration for the earn-out in connection with the acquisition of Universal Robots was increased by $7.8 million primarily due to an increase in forecasted revenues and decrease in discount rate.

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