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Notes and Accounts Receivable, Net - Movements of Allowance for Doubtful Receivables (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of financial assets [line items]      
Beginning balance $ 7.3 $ 471.8 $ 480.1
Reversal/Write-off     (8.1)
Provision (Reversal) 318.2    
Effect of exchange rate changes (0.2)   (0.2)
Ending balance 325.3 7.3 471.8
IFRS 9 [member]      
Disclosure of financial assets [line items]      
Beginning balance $ 7.3 227.0  
Provision (Reversal)   (219.7)  
Ending balance   7.3 227.0
Individually assessed for Impairment [member]      
Disclosure of financial assets [line items]      
Beginning balance     1.8
Reversal/Write-off     (1.8)
Collectively assessed for impairment [member]      
Disclosure of financial assets [line items]      
Beginning balance   $ 471.8 478.3
Reversal/Write-off     (6.3)
Effect of exchange rate changes     (0.2)
Ending balance     $ 471.8