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Retirement Benefit Plans - Additional Information (Detail)
$ in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2019
TWD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Dec. 31, 2019
USD ($)
Disclosure of defined benefit plans [line items]          
Percentage of monthly pension contributions 6.00% 6.00% 6.00% 6.00%  
Pension costs $ 277,041 $ 9,263 $ 291,333 $ 266,267  
Expected contribution to defined benefit plans 533,777 17,846 484,247    
Pension plans for executive managers [member]          
Disclosure of defined benefit plans [line items]          
Pension costs 11,567 $ 387 11,137 $ 3,171  
Accrued pension liabilities $ 335,109   $ 320,542   $ 11,204
Bottom of range [member]          
Disclosure of defined benefit plans [line items]          
Average duration of the defined benefit obligation 10 years 10 years 9 years    
Top of range [member]          
Disclosure of defined benefit plans [line items]          
Average duration of the defined benefit obligation 14 years 14 years 15 years