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Accounts and notes receivable - Aging of Accounts Receivable which are Past Due but not Impaired (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Jan. 01, 2018
TWD ($)
Disclosure of financial assets [Line Items]        
Accounts and notes receivable $ 4,453,669 $ 148,902 $ 4,747,288  
Accounts and notes receivable [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable 4,454,255   4,747,834 $ 4,013,705
Accounts and notes receivable [member] | Current [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable 4,440,846   4,596,895  
Accounts and notes receivable [member] | Within 1 month [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable 13,733   18,807  
Accounts and notes receivable [member] | 1 - 2 months [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable $ 441   131,787  
Accounts and notes receivable [member] | 2 - 3 months [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable     1,436  
Accounts and notes receivable [member] | 3 - 4 months [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable     180  
Accounts and notes receivable [member] | Over 4 months [member]        
Disclosure of financial assets [Line Items]        
Accounts and notes receivable     $ 324