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Subsidiaries - Additional Information (Detail) - TWD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2018 | Jan. 01, 2015 | Dec. 31, 2014
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Pro-forma revenue |  |  | $ 232,187 |  |  |
Pro-forma net income |  |  | $ 42,774 |  |  |
CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Equity ownership percentage | 67.00% | 69.00% |  |  |  |
Chunghwa Investment Co., Ltd. (CHI) [Member] | Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Equity ownership percentage | 38.30% |  | 38.30% |  |  |
Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Equity ownership percentage |  |  | 29.31% |  | 28.18% |
Senao International Co., Ltd. (SENAO) [Member] | Youth Co., Ltd. (Youth) [Member] |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Equity ownership percentage |  |  | 89.48% |  |  | 70.00%
Discount rate applied to cash flow projections | 14.80% | 14.60% | 16.30% |  |  |
Impairment loss | $ 9 | $ 0 | $ 0 |  |  |
Chunghwa Telecom Co., Ltd. [Member] | Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Equity ownership percentage | 28.93% | 29.00% |  |  |  |
Chunghwa Telecom Co., Ltd. [Member] | CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |
Equity ownership percentage | 70.43% |  |  | 69.31% |  |

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