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Additional Information of Expenses by Nature - Additional Information (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Expense by nature [line items]      
Profit sharing bonus to employees, amount $ 23,165.7 $ 23,570.0 $ 23,019.1
Compensation to Directors [member]      
Expense by nature [line items]      
Approved profit sharing bonus in cash 360.4 349.3 368.9
Bonus to Employees [member]      
Expense by nature [line items]      
Approved profit sharing bonus in cash $ 23,165.7 $ 23,570.0 $ 23,019.1
Top of range [member] | Compensation to Directors [member]      
Expense by nature [line items]      
Profit sharing bonus, percentage 0.30% 0.30% 0.30%
Bottom of range [member] | Bonus to Employees [member]      
Expense by nature [line items]      
Profit sharing bonus, percentage 1.00% 1.00% 1.00%