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Segment Information - Main Products and Service Revenues (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
TWD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
TWD ($)
Dec. 31, 2015
TWD ($)
Disclosure of products and services [Line Items]        
Revenues $ 227,514 $ 7,676 $ 229,991 $ 231,795
Mobile services [Member]        
Disclosure of products and services [Line Items]        
Revenues 75,823   78,788 80,867
Local telephone and domestic long distance telephone services [Member]        
Disclosure of products and services [Line Items]        
Revenues 32,247   34,531 36,690
Sales of product [Member]        
Disclosure of products and services [Line Items]        
Revenues 37,649   35,377 36,509
Broadband access and domestic leased line services [Member]        
Disclosure of products and services [Line Items]        
Revenues 22,950   23,315 23,711
Data communications internet services [Member]        
Disclosure of products and services [Line Items]        
Revenues 21,143   20,906 17,455
International network and leased telephone services [Member]        
Disclosure of products and services [Line Items]        
Revenues 9,328   10,634 11,319
Others [Member]        
Disclosure of products and services [Line Items]        
Revenues $ 28,374   $ 26,440 $ 25,244