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Subsidiaries - Detailed Information of Equity Transactions (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
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
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests     (1,750)
Differences arising from equity transactions     802
Additional paid-in capital - arising from changes in equities of subsidiaries     802
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 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  
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests   (330)  
Differences arising from equity transactions   712  
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  
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 $ 19 35  
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (20) (24)  
Differences arising from equity transactions (1) 11  
Additional paid-in capital - arising from changes in equities of subsidiaries $ (1) $ 11