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Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Balance |  | $ 1,522,354 | $ 1,953,646 | $ 1,828,659
Other comprehensive income (loss) before reclassifications, net of tax |  | (4,976) | (30,552) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | (838) | 1,092 |
Other comprehensive (loss) income |  | (5,814) | (29,460) | 38,990
Reclassification of tax effects resulting from the Tax Reform Act | [1] |  | 769 |
Reclassification of unrealized gains on equity securities,net of tax |  | (690) | 1,337 | (441)
Balance |  | 1,480,158 | 1,522,354 | 1,953,646
ASU 2016-01 |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Reclassification of unrealized gains on equity securities,net of tax | [2] |  | (3,125) |
Foreign Currency Translation Reclassification Adjustments |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Balance |  | (12,523) | 15,919 |
Other comprehensive income (loss) before reclassifications, net of tax |  | (10,991) | (28,442) |
Other comprehensive (loss) income |  | (10,991) | (28,442) |
Balance |  | (23,514) | (12,523) | 15,919
Unrealized Gains (Losses) on Marketable Securities |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Balance |  | (1,845) | 1,362 |
Other comprehensive income (loss) before reclassifications, net of tax |  | 6,015 | (2,110) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | (690) | 1,337 |
Other comprehensive (loss) income |  | 5,325 | (773) |
Reclassification of tax effects resulting from the Tax Reform Act | [1] |  | 691 |
Balance |  | 3,480 | (1,845) | 1,362
Unrealized Gains (Losses) on Marketable Securities | ASU 2016-01 |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Reclassification of unrealized gains on equity securities,net of tax | [2] |  | (3,125) |
Amortization of Prior Service Credit |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Balance |  | 1,328 | 1,495 |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | (148) | (245) | (272)
Other comprehensive (loss) income |  | (148) | (245) |
Reclassification of tax effects resulting from the Tax Reform Act | [1] |  | 78 |
Balance |  | 1,180 | 1,328 | 1,495
Accumulated Other Comprehensive Income (Loss) |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Balance |  | (13,040) | 18,776 | (20,214)
Amounts reclassified from accumulated other comprehensive income, net of tax |  | (838) | 1,092 | (713)
Other comprehensive (loss) income |  | (5,814) | (29,460) | 38,990
Reclassification of tax effects resulting from the Tax Reform Act |  |  | 769 |
Balance |  | $ (18,854) | $ (13,040) | $ 18,776
[1] | In the year ended December 31, 2018, Teradyne early adopted ASU 2018-02, “Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.” As a result, the stranded tax effects resulting from the Tax Reform Act enacted in December 2017 were reclassified from accumulated other comprehensive income to retained earnings.
[2] | In the year ended December 31, 2018, Teradyne adopted ASU 2016-01, “Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.” See Note B: “Accounting Policies.”

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