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Notes and Accounts Receivable, Net - Movements of Allowance for Doubtful Receivables (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of financial assets [line items]    
Beginning balance $ 471.8 $ 480.1
Effect of retrospective application of IFRS 9 (244.8)  
Reversal/Write-off   (8.1)
Provision (Reversal) (219.7)  
Effect of exchange rate changes   (0.2)
Ending balance 7.3 471.8
IAS 39 [Member]    
Disclosure of financial assets [line items]    
Beginning balance 471.8  
Ending balance   471.8
IFRS 9 [member]    
Disclosure of financial assets [line items]    
Beginning balance 227.0  
Ending balance   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