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Retirement Benefit Plans - Amounts Included in Consolidated Balance Sheets from Obligation in Respect of Defined Benefit Plans (Detail)
$ in Millions, $ in Millions
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Dec. 31, 2014
TWD ($)
Disclosure Of Defined Benefit Plans [Abstract]          
Present value of funded defined benefit obligation $ 37,663   $ 34,572    
Fair value of plan assets (34,972)   (33,954)    
Funded status - deficit 2,691   618    
Net defined benefit liabilities 2,704 $ 91 1,537    
Net defined benefit assets (13) $ 0 (919)    
Defined benefit obligation $ 2,691   $ 618 $ 7,088 $ 6,462