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Summary of Significant Accounting Policies - Additional information (Detail) | 12 Months Ended |
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Dec. 31, 2017 | Dec. 31, 2016
Disclosure of subsidiaries [Line Items] |  |
Income tax percentage on undistributed earnings | 10.00% |
CHIEF Telecom Inc. (CHIEF) [Member] |  |
Disclosure of subsidiaries [Line Items] |  |
Percentage of Ownership | 67.00% | 69.00%
Chunghwa Investment Co., Ltd. (CHI) [Member] | CHIEF Telecom Inc. (CHIEF) [Member] |  |
Disclosure of subsidiaries [Line Items] |  |
Percentage of Ownership | 3.53% |
Chunghwa Investment Co., Ltd. (CHI) [Member] | Senao International Co., Ltd. (SENAO) [Member] |  |
Disclosure of subsidiaries [Line Items] |  |
Percentage of Ownership | 0.40% |

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