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Summary of Significant Accounting Policies - Detail Information of Subsidiaries (Parenthetical) (Detail) | Mar. 31, 2018 | Dec. 31, 2017 | Nov. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 01, 2015
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CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 67.00% |  |  | 69.00% |  |
Chunghwa Leading Photonics Tech Co., Ltd. (CLPT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 75.00% |  |  | 75.00% |  |
Chunghwa Telecom (Thailand) Co., Ltd. (“CHTT”) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 100.00% |  | 100.00% |  |  |
Chunghwa Telecom Security Co Ltd [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 80.00% |  |  |  |  |
Chunghwa Telecom Co., Ltd. [Member] | Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 28.93% |  |  | 29.00% |  |
Chunghwa Telecom Co., Ltd. [Member] | CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership | 69.31% | 70.43% |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  |  |  |  |  | 29.31% | 28.18%
Senao International Co., Ltd. (SENAO) [Member] | SENYOUNG Insurance Agent Co., Ltd. [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 100.00% | 100.00% |  |  |  |
Chunghwa Investment Co., Ltd. (CHI) [Member] | Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership |  | 38.30% |  |  |  | 38.30% |
Light Era Development Co., Ltd. (LED) [Member] | Taoyuan Asia Silicon Valley Innovation Co., Ltd [Member] |  |  |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |  |  |
Percentage of Ownership | 60.00% |  |  |  |  |  |

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