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Retirement Benefit Plans - Summary of Maturity Analysis of Undiscounted Pension Benefit (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2018
USD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2017
TWD ($)
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit $ 508,920 $ 15,578,025 $ 18,350,395
Less than 1 year [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit 12,042 368,592 291,152
1-5 years [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit 61,638 1,886,738 1,551,496
More than 5 years [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit $ 435,240 $ 13,322,695 $ 16,507,747