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Segment Information - Main Products and Service Revenues (Detail) $ in Millions, $ in Millions | 12 Months Ended
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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

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