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Retirement Benefit Plans - Summary of Maturity Analysis of Undiscounted Pension Benefit (Detail)
$ in Thousands, $ in Thousands
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit $ 18,350,395 $ 619,109 $ 19,095,234
Less than 1 year [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit 291,152 9,823 292,100
1-5 years [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit 1,551,496 52,345 1,673,549
More than 5 years [member]      
Disclosure of defined benefit plans [line items]      
Maturity analysis of undiscounted pension benefit $ 16,507,747 $ 556,941 $ 17,129,585