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Financial Instruments - Additional Information (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items]      
Increase decrease in fixed income investment fair value $ (3,143.6) $ (3,517.4) $ (2,697.8)
Percentage of accounts receivable from ten largest customers 79.00% 79.00%  
Expected credit loss decrease increase $ 1.1 $ 0.6 $ (1.1)
Level 3 [member]      
Disclosure of detailed information about financial instruments [line items]      
Discount for lack of marketability percentage 10.00% 10.00%  
Decrease in fair value due to lack of marketability increase by 1% $ (39.0) $ (34.8)  
Percentage increase in the discount for lack of marketability 1.00% 1.00%  
Equity Price Risk [member]      
Disclosure of detailed information about financial instruments [line items]      
Assumed change in risk (10.00%) (10.00%) (10.00%)
Assumed change in risk, monetary effect $ (446.5) $ (401.9) $ (427.1)
Currency Risk [member]      
Disclosure of detailed information about financial instruments [line items]      
Assumed change in risk (10.00%) (10.00%) (10.00%)
Assumed change in risk, monetary effect $ (897.7) $ (2,137.3) $ (506.4)
Currency Risk [member] | Other comprehensive income [member]      
Disclosure of detailed information about financial instruments [line items]      
Assumed change in risk, monetary effect   $ (107.7) $ (315.6)
Interest rate risk [member] | Fixed Income Securities [member]      
Disclosure of detailed information about financial instruments [line items]      
Assumed change in risk 1.00% 1.00% 1.00%
Assumed change in risk, monetary effect $ (3,143.6) $ (3,516.6) $ (2,450.0)
Decrease in net income   $ (0.8) $ (247.8)