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Segment Information - Main Products and Service Revenues (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Disclosure of products and services [Line Items]        
Revenues $ 207,609 $ 7,393 $ 207,520 $ 215,483
Mobile services [Member]        
Disclosure of products and services [Line Items]        
Revenues 56,724   58,703 63,906
Local telephone and domestic long distance telephone services [Member]        
Disclosure of products and services [Line Items]        
Revenues 26,475   27,929 29,996
Sales of products [Member]        
Disclosure of products and services [Line Items]        
Revenues 39,391   41,593 41,289
Broadband access and domestic leased line services [Member]        
Disclosure of products and services [Line Items]        
Revenues 22,420   22,116 22,453
Data communications internet services [Member]        
Disclosure of products and services [Line Items]        
Revenues 21,447   21,003 21,137
International network and leased line services revenue [Member]        
Disclosure of products and services [Line Items]        
Revenues 3,884   7,066 8,724
Others [Member]        
Disclosure of products and services [Line Items]        
Revenues $ 37,268   $ 29,110 $ 27,978