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Retirement Benefit Plans - Additional Information (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of defined benefit plans [line items]      
Employer contribution percentage, defined contribution plans 6.00%    
Defined contribution plans expense $ 2,609.7 $ 2,568.9 $ 2,369.9
Employer contribution percentage, defined benefit plans 2.00%    
Defined benefit plans contribution in the next year $ 230.9    
Weighted average duration of the defined benefit obligation 10 years    
Actuarial assumption of discount rates [member]      
Disclosure of defined benefit plans [line items]      
Percentage of decrease in discount rate 0.50%    
Amount of increase in defined benefit obligation $ 725.0 921.8  
Actuarial assumption of expected rates of salary increases [member]      
Disclosure of defined benefit plans [line items]      
Percentage of increase in salary rate 0.50%    
Amount of increase in defined benefit obligation $ 706.5 $ 901.6