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Subsidiaries - Detailed Information of Equity Transactions (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Senao International Co Ltd Transferred Its Treasury Stock [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from noncontrolling interests     $ 327
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests     (272)
Differences arising from equity transactions     55
Additional paid-in capital - arising from changes in equities of subsidiaries     55
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 noncontrolling interests     133
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests     (19)
Differences arising from equity transactions     114
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
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 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 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 noncontrolling interests $ 75 $ 19 35
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (49) (20) (24)
Differences arising from equity transactions 26 (1) 11
Additional paid-in capital - arising from changes in equities of subsidiaries 26 $ (1) 11
SENAO not Proportionately participating in the Capital Increase of Youth [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from noncontrolling interests 0   0
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests 0   0
Differences arising from equity transactions 0   0
Additional paid-in capital - arising from changes in equities of subsidiaries 0   $ 0
IISI Share-Based Payment [Member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Cash consideration received from noncontrolling interests 7    
The proportionate share of the carrying amount of the net assets of the subsidiary transferred (to) from noncontrolling interests (7)    
Differences arising from equity transactions 0    
Additional paid-in capital - arising from changes in equities of subsidiaries $ 0