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Subsidiaries - Detailed Information of Equity Transactions (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Chunghwa Investment Co Ltd Disposed Some Shares Of Chunghwa Precision Test Tech Co Ltd      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests $ 1,042   $ 83
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (330)   (25)
Differences arising from equity transactions 712   58
Additional paid-in capital - difference between consideration received or paid and the carrying amount of the subsidiaries? net assets upon actual disposal or acquisition 712   58
Chunghwa Investment Co Ltd Did Not Participate In The Capital Increase Of Chunghwa Precision Test Tech Co Ltd [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests   $ 2,552 1,175
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests   (1,750) (786)
Differences arising from equity transactions   802 389
Additional paid-in capital - arising from changes in equities of subsidiaries   802 $ 389
Senao International Co Ltd Transferred Its Treasury Stock [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests 327 164  
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (272) (137)  
Differences arising from equity transactions 55 27  
Additional paid-in capital - arising from changes in equities of subsidiaries 55 27  
Chunghwa Telecom Co Ltd and Chunghwa Investment Co Ltd Disposed Some Shares of CHIEF Telecom Inc [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests 133 106  
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (19) (29)  
Differences arising from equity transactions 114 77  
Additional paid-in capital - difference between consideration received or paid and the carrying amount of the subsidiaries? net assets upon actual disposal or acquisition 114 $ 77  
SENAO not Proportionately participating in the Capital Increase of Youth [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests 0    
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests 0    
Differences arising from equity transactions 0    
Additional paid-in capital - difference between consideration received or paid and the carrying amount of the subsidiaries? net assets upon actual disposal or acquisition 0    
Additional paid-in capital - arising from changes in equities of subsidiaries 0    
Chunghwa Telecom Co Ltd and Chunghwa Investment Co Ltd Did Not Participate in the Capital Increase of CHIEF Telecom Inc [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests 1,477    
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (700)    
Differences arising from equity transactions 777    
Additional paid-in capital - arising from changes in equities of subsidiaries 777    
Share-Based Payment of CHIEF Telecom Inc [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from (paid to)Noncontrolling interests 35    
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (24)    
Differences arising from equity transactions 11    
Additional paid-in capital - arising from changes in equities of subsidiaries $ 11