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Changes in Accumulated Other Comprehensive (Loss) Income (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss), tax	$ (667)	$ (235)	$ (92)
Foreign currency translation adjustments, tax	0	0	0
Unrealized (losses) gains on marketable securities, tax	1,910	946	(521)
Retirement plans prior service benefit, tax	(1,126)	(1,124)	(1,081)
Foreign Currency Translation Reclassification Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	0	0
Amounts reclassified from accumulated other comprehensive income (loss), tax	0	0
Other comprehensive income (loss), tax	0	0
Unrealized (Losses) Gains on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	1,629	1,659
Amounts reclassified from accumulated other comprehensive income (loss), tax	(665)	(192)	(21)
Other comprehensive income (loss), tax	964	1,467
Amortization of Prior Service Credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax	0	0
Amounts reclassified from accumulated other comprehensive income (loss), tax	(2)	(43)	$ (71)
Other comprehensive income (loss), tax	$ (2)	$ (43)

X

- Definition

The tax effect from accumulated appreciation or loss, in value of the total of unsold securities at the end of an accounting period.

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The tax effect from the total of net (gain) loss, prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost.

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The tax effect from accumulated adjustment, that results from the process of translating subsidiary financial statements and foreign equity investments into the reporting currency from the functional currency of the reporting entity, net of reclassification of realized foreign currency translation gains (losses).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) before reclassification adjustment from accumulated other comprehensive income (loss).

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Amount of tax expense (benefit) allocated to other comprehensive income (loss).

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