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Additional Information of Expenses by Nature - Additional Information (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Expense by nature [line items]      
Profit sharing bonus to employees, amount $ 23,570.0 $ 23,019.1 $ 22,418.3
Director 1 [member]      
Expense by nature [line items]      
Approved profit sharing bonus in cash 349.3 368.9 376.4
Employees [member]      
Expense by nature [line items]      
Approved profit sharing bonus in cash $ 23,570.0 $ 23,019.1 $ 22,418.3
Top of range [member] | Director 1 [member]      
Expense by nature [line items]      
Profit sharing bonus, percentage 0.30% 0.30% 0.30%
Bottom of range [member] | Employees [member]      
Expense by nature [line items]      
Profit sharing bonus, percentage 1.00% 1.00% 1.00%