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Leasing arrangements - leasee - summary of carrying amount of right-of-use assets and the depreciation expenses (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2020
USD ($)
Disclosure of quantitative information about right-of-use assets [line items]      
Carrying amount $ 859,069 $ 687,068 $ 30,594
Depreciation expenses 76,526 36,099  
Land [member]      
Disclosure of quantitative information about right-of-use assets [line items]      
Carrying amount 636,261 669,967  
Depreciation expenses 20,938 22,657  
Buildings [member]      
Disclosure of quantitative information about right-of-use assets [line items]      
Carrying amount 19,044 15,043  
Depreciation expenses 7,819 7,113  
Machinery and equipment [member]      
Disclosure of quantitative information about right-of-use assets [line items]      
Carrying amount 203,249    
Depreciation expenses 46,225 4,520  
Others [member]      
Disclosure of quantitative information about right-of-use assets [line items]      
Carrying amount 515 2,058  
Depreciation expenses $ 1,544 $ 1,809