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Retirement Benefit Plans - Summary of Maturity Analysis of Undiscounted Pension Benefit (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit $ 15,377,190 $ 547,620 $ 14,720,029
Less than 1 year [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit 498,092 17,738 422,067
1-5 years [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit 2,553,522 90,937 2,081,540
More than 5 years [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit $ 12,325,576 $ 438,945 $ 12,216,422