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.”