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Intangible Assets - Additional Information (Detail) - TWD ($) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Nov. 30, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
4G concession fees | $ 10,935 |  |  |
Senao Networks, Inc. ("SNI") [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Discount rate |  | 12.30% | 13.70% | 14.80%
Senao Networks, Inc. ("SNI") [Member] | Goodwill [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Impairment loss |  | $ 9 | $ 0 | $ 9
Senao Networks, Inc. ("SNI") [Member] | Certain licensed contract [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Impairment loss |  |  | $ 51 |
Computer software [Member] | Bottom of range [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Estimated useful lives |  | 1 year |  |
Computer software [Member] | Top of Range [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Estimated useful lives |  | 10 years |  |
Others [Member] | Bottom of range [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Estimated useful lives |  | 1 year |  |
Others [Member] | Top of Range [Member] |  |  |  |
Disclosure of detailed information about intangible assets [Line Items] |  |  |  |
Estimated useful lives |  | 20 years |  |

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