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Subsidiaries - Additional Information (Detail) $ in Millions, $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($) | Sep. 30, 2020 | Jul. 31, 2020 | Jul. 01, 2020 TWD ($)
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Revenue | $ 207,609 | $ 7,393 | $ 207,520 | $ 215,483 |  |  |
IISI and its Subsidiaries [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Fair value of trade notes and accounts receivable acquired |  |  |  |  |  |  | $ 165
Gross contractual amount |  |  |  |  |  |  | 167
Best estimates of contractual cash flows not expected to be collected as of acquisition date |  |  |  |  |  |  | $ 2
Revenue | 208,605 |  |  |  |  |  |
Profit | $ 34,747 |  |  |  |  |  |
International Integrated Systems, Inc. ("IISI") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Equity ownership percentage | 51.00% | 51.00% | 0.00% |  | 51.20% | 51.54% |
Percentage of additional ownership interest obtained |  |  |  |  |  |  | 20.38%
Chunghwa Investment Co., Ltd. ("CHI") [Member] | Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Equity ownership percentage | 34.25% | 34.25% | 34.25% |  |  |  |
Chunghwa Telecom Co., Ltd. and Chunghwa Investment Co., Ltd. (CHI) [Member] | CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Equity ownership percentage | 59.08% | 59.08% | 59.75% |  |  |  |
Chunghwa Telecom Co., Ltd. and Chunghwa Investment Co., Ltd. (CHI) [Member] | Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Equity ownership percentage | 28.18% | 28.18% | 28.18% |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | Youth Co., Ltd. ("Youth") [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Equity ownership percentage | 96.00% | 96.00% | 93.00% |  |  |  |

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