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v3.21.1

Intangible Assets - Additional Information (Detail) - TWD ($) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 29, 2020 | Oct. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
5G spectrum bidding amount |  | $ 1,000 |  |  |
Additions-acquired separately | $ 48,373 |  |  |  |
Senao Networks, Inc. ("SNI") [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Discount rate |  |  | 12.10% | 12.30% | 13.70%
Senao Networks, Inc. ("SNI") [Member] | Goodwill [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Impairment loss |  |  | $ 9 | $ 9 | $ 0
Senao Networks, Inc. ("SNI") [Member] | Certain licensed contract [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Impairment loss |  |  |  |  | $ 51
Bottom of range [Member] | Computer software [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Estimated useful lives |  |  | 1 year |  |
Bottom of range [Member] | Others [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Estimated useful lives |  |  | 1 year |  |
Top of Range [Member] | Computer software [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Estimated useful lives |  |  | 10 years |  |
Top of Range [Member] | Others [Member] |  |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |  |
Estimated useful lives |  |  | 20 years |  |

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