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Summary of Significant Accounting Policies - Detail Information of Subsidiaries (Parenthetical) (Detail) | Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Nov. 30, 2017 | Mar. 31, 2017
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CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 57.00% |  | 67.00% |  |
Chunghwa Telecom (Thailand) Co., Ltd. ("CHTT") [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 100.00% |  | 100.00% |  | 100.00%
Chunghwa Telecom Security Co Ltd [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 80.27% |  | 80.00% |  |
Chunghwa Telecom Co., Ltd. [Member] | Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 28.18% |  | 29.00% |  |
Chunghwa Telecom Co., Ltd. [Member] | CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 60.23% |  | 70.43% |  |
Youth Co., Ltd. (Youth) [Member] | Senao International Co., Ltd. (SENAO) [Member] | Bottom of Range [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 89.00% |  |  |  |
Youth Co., Ltd. (Youth) [Member] | Senao International Co., Ltd. (SENAO) [Member] | Top of Range [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 93.00% |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | SENYOUNG Insurance Agent Co., Ltd. [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 100.00% |  | 100.00% | 100.00% |
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% | 60.00% |  |  |
Chunghwa Investment Co., Ltd. (CHI) [Member] | Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] |  |  |  |  |
Disclosure of subsidiaries [Line Items] |  |  |  |  |
Percentage of Ownership | 34.25% |  |  |  |

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