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Notes and Accounts Receivable, Net - Movements of Allowance for Doubtful Receivables (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of financial assets [line items]      
Beginning balance $ 480.1 $ 488.2 $ 486.7
Provision   0.3 33.4
Reversal/Write-off (8.1) (8.4) (33.8)
Effect of acquisition of subsidiary     1.8
Effect of exchange rate changes (0.2)   0.1
Ending balance 471.8 480.1 488.2
Individually assessed for Impairment [member]      
Disclosure of financial assets [line items]      
Beginning balance 1.8 10.2 8.1
Provision     28.6
Reversal/Write-off (1.8) (8.4) (29.1)
Effect of acquisition of subsidiary     1.8
Effect of exchange rate changes     0.8
Ending balance   1.8 10.2
Collectively assessed for impairment [member]      
Disclosure of financial assets [line items]      
Beginning balance 478.3 478.0 478.6
Provision   0.3 4.8
Reversal/Write-off (6.3)   (4.7)
Effect of exchange rate changes (0.2)   (0.7)
Ending balance $ 471.8 $ 478.3 $ 478.0