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Revenue - Summary of Disaggregation of Revenue (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers $ 206,396 $ 206,360 $ 214,461
Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 68,804 65,066 66,103
Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 90,143 95,390 100,877
Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 31,809 29,834 29,650
International Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 8,682 11,475 13,424
Other [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 6,958 4,595 4,407
Mobile services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 56,724 58,703 63,906
Mobile services [Member] | Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 56,724 58,703 63,906
Sales of product [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 39,391 41,593 41,289
Sales of product [Member] | Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 2,214 1,957 1,731
Sales of product [Member] | Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 32,112 35,545 35,702
Sales of product [Member] | Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 107 41 4
Sales of product [Member] | International Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 313 265 251
Sales of product [Member] | Other [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 4,645 3,785 3,601
Local telephone and domestic long distance telephone services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 26,475 27,929 29,996
Local telephone and domestic long distance telephone services [Member] | Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 26,475 27,929 29,996
Broadband access and domestic leased line services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 22,420 22,116 22,453
Broadband access and domestic leased line services [Member] | Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 22,420 22,116 22,453
Data communications internet services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 21,447 21,003 21,137
Data communications internet services [Member] | Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 21,447 21,003 21,137
International network and leased line services      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 3,884 7,066 8,724
International network and leased line services | International Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 3,884 7,066 8,724
Others [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 36,055 27,950 26,956
Others [Member] | Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 17,695 13,064 11,923
Others [Member] | Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 1,307 1,142 1,269
Others [Member] | Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 10,255 8,790 8,509
Others [Member] | International Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 4,485 4,144 4,449
Others [Member] | Other [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers $ 2,313 $ 810 $ 806