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Inventories - Summary of Inventories (Detail) - TWD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Disclosure of inventories [Line Items] |  |
Raw materials | $ 1,767,642 | $ 1,778,835
Gross carrying amount [member] |  |
Disclosure of inventories [Line Items] |  |
Raw materials | 1,831,140 | 1,814,992
Allowance for impairment losses [member] |  |
Disclosure of inventories [Line Items] |  |
Raw materials | $ (63,498) | $ (36,157)

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