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Revenue - Summary of Disaggregation of Revenue (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers $ 208,412 $ 206,396 $ 206,360
Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 64,144 68,804 65,066
Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 94,684 90,143 95,390
Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 32,837 31,809 29,834
International Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 8,625 8,682 11,475
Other [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 8,122 6,958 4,595
Mobile services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 58,048 56,724 58,703
Mobile services [Member] | Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 58,048 56,724 58,703
Sales of product [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 42,907 39,391 41,593
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 2,173 2,214 1,957
Sales of product [Member] | Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 35,414 32,112 35,545
Sales of product [Member] | Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 128 107 41
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 12 313 265
Sales of product [Member] | Other [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 5,180 4,645 3,785
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 25,704 26,475 27,929
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 25,704 26,475 27,929
Broadband access and domestic leased line services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 23,012 22,420 22,116
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 23,012 22,420 22,116
Data communications internet services [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 22,281 21,447 21,003
Data communications internet services [Member] | Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 22,281 21,447 21,003
International network and leased line services      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 4,367 3,884 7,066
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 4,367 3,884 7,066
Others [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 32,093 36,055 27,950
Others [Member] | Domestic Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 13,255 17,695 13,064
Others [Member] | Mobile Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 1,222 1,307 1,142
Others [Member] | Internet Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 10,428 10,255 8,790
Others [Member] | International Fixed Communications Business [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers 4,246 4,485 4,144
Others [Member] | Other [Member]      
Disclosure Of Disaggregation Of Revenue From Contracts With Customers [Line Items]      
Revenue from contracts with customers $ 2,942 $ 2,313 $ 810