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Lease Arrangements - Summary of Right-of-use Assets (Details)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Dec. 31, 2020
USD ($)
Disclosure Of Quantitative Information About Rightofuse Assets [Line Items]        
Right-of-use assets $ 11,009 $ 11,364   $ 392
Additions to right-of-use assets 3,796 3,803    
Depreciation charge for right-of-use assets        
Depreciation, right-of-use assets 3,931 3,968 $ 0  
Handsets Base Stations [Member]        
Disclosure Of Quantitative Information About Rightofuse Assets [Line Items]        
Right-of-use assets 7,096 6,845    
Depreciation charge for right-of-use assets        
Depreciation, right-of-use assets 2,729 2,728    
Others [Member]        
Disclosure Of Quantitative Information About Rightofuse Assets [Line Items]        
Right-of-use assets 1,708 1,916    
Depreciation charge for right-of-use assets        
Depreciation, right-of-use assets 786 821    
Equipment [Member]        
Disclosure Of Quantitative Information About Rightofuse Assets [Line Items]        
Right-of-use assets 2,205 2,603    
Depreciation charge for right-of-use assets        
Depreciation, right-of-use assets $ 416 $ 419