Reclassifications Out of Accumulated Other Comprehensive Income to Statements of Operations (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, tax	$ 235	$ 92	$ 451
Unrealized (Losses) Gains on Marketable Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, tax	192	21	297
Amortization of Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, tax	$ 43	$ 71	$ 154

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL34724391-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109238882&loc=d3e39076-109324

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