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Inventories - Summary of Inventories (Detail) - TWD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Disclosure of inventories [Line Items] |  |
Raw materials | $ 2,102,075 | $ 1,767,642
Gross carrying amount [member] |  |
Disclosure of inventories [Line Items] |  |
Raw materials | 2,181,890 | 1,831,140
Allowance for impairment losses [member] |  |
Disclosure of inventories [Line Items] |  |
Raw materials | $ (79,815) | $ (63,498)

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