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Share-based Payments Arrangement - Additional Information (Detail) $ / shares in Units, $ / shares in Units, $ in Millions |  |  |  |  |  |  |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 20, 2019 $ / shares shares | Oct. 31, 2018 $ / shares shares | Dec. 19, 2017 $ / shares shares | Oct. 22, 2015 shares | Jan. 31, 2014 shares | Aug. 31, 2013 shares | May 07, 2013 shares | Mar. 31, 2018 shares | Dec. 31, 2020 TWD ($) $ / shares | Dec. 31, 2020 TWD ($) $ / shares | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) $ / shares | Dec. 31, 2018 TWD ($) $ / shares shares | Dec. 31, 2020 $ / shares | Jul. 31, 2020 $ / shares | Jun. 30, 2019 $ / shares | Aug. 31, 2018 $ / shares | Jul. 31, 2018 $ / shares | Jun. 30, 2018 $ / shares | Dec. 31, 2017
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs |  |  |  |  |  |  |  |  |  | $ 8,000,000 | $ 0 | $ 2,000,000 | $ 17,000,000 |  |  |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common share each option is eligible | shares |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period |  |  |  |  |  |  | six years |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise Price |  |  |  |  |  |  |  |  | $ 66.20 | $ 66.20 |  |  |  |  |  |  |  | $ 66.20 | $ 70.70 |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 | $ 0 |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 0 | 0 |  | 0 | 5,318,000 | 0 |  |  |  |  |  | 5,926,000
Exercisable price |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 66.20 |  |  |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | Stock Option Granted in April Member 2018 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,600,000 |  |  |  |  |  |  |
Number of treasury stock transfer to its specific employees | shares |  |  |  |  |  |  |  |  |  |  |  |  | 6,658,000 |  |  |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | 2 years after grant date [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  | two years |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting percentage |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | 3 years after grant date [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  | three years |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting percentage |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | 4 years after grant date [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  | four years |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting percentage |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |
CHIEF Telecom Inc. (CHIEF) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common share each option is eligible | shares |  | 1,000 | 1,000 | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |
Shares issued | shares |  |  |  |  |  |  |  | 7,842,000 |  |  |  |  |  |  |  |  |  |  |  |
Reserve for employee subscription | shares |  |  |  |  |  |  |  | 1,176,000 |  |  |  |  |  |  |  |  |  |  |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Granted on October 22, 2015 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period |  |  |  |  |  |  |  |  |  | five years | five years |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  |  |  |  | two years | two years |  |  |  |  |  |  |  |  |
Exercise Price |  |  |  |  |  |  |  |  | $ 34.40 | $ 34.40 |  | $ 34.40 |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 300,000 | $ 1,000,000 |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 0 | 0 |  | 314.25 | 882.75 | 0 |  |  |  |  |  | 1,936.00
Exercisable price |  |  |  |  |  |  |  |  | $ 0 | $ 0 |  | $ 34.40 | $ 34.40 |  |  |  |  |  |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Granted on December 19, 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period |  |  |  |  |  |  |  |  |  | five years | five years |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  |  |  |  | two years | two years |  |  |  |  |  |  |  |  |
Exercise Price |  |  | $ 147.00 |  |  |  |  |  | $ 132.70 | $ 132.70 |  | $ 135.60 |  |  | $ 132.70 | $ 135.60 | $ 140.60 |  | $ 144.10 |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 200,000 |  | $ 600,000 | $ 600,000 |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 427.50 | 427.50 |  | 897.00 | 925.00 | 427.50 |  |  |  |  |  | 950.00
Exercisable price |  |  |  |  |  |  |  |  | $ 132.70 | $ 132.70 |  | $ 135.60 | $ 0 |  |  |  |  |  |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Granted on October 31, 2018 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period |  |  |  |  |  |  |  |  |  | five years | five years |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  |  |  |  | two years | two years |  |  |  |  |  |  |  |  |
Exercise Price |  | $ 147.00 |  |  |  |  |  |  | $ 138.70 | $ 138.70 |  | $ 141.70 |  |  | $ 138.70 | $ 141.70 |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 300,000 |  | $ 600,000 | $ 100,000 |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 21.00 | 21.00 |  | 46.00 | 50.00 | 21.00 |  |  |  |  |  | 0
Exercisable price |  |  |  |  |  |  |  |  | $ 0 | $ 0 |  | $ 0 | $ 0 |  |  |  |  |  |  |
CHIEF Telecom Inc. (CHIEF) [Member] | Granted on November 13, 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period |  |  |  |  |  |  |  |  |  | five years | five years |  |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  |  |  |  |  | two years | two years |  |  |  |  |  |  |  |  |
Exercise Price |  |  |  |  |  |  |  |  | $ 206.00 | $ 206.00 |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 1,000,000 |  |  |  |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 200.00 | 200.00 |  | 0 |  | 200.00 |  |  |  |  |  |
Exercisable price |  |  |  |  |  |  |  |  | $ 0 | $ 0 |  |  |  |  |  |  |  |  |  |
CHT Security Co., Ltd. (CHTSC) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common share each option is eligible | shares | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period | five years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period | one year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise Price |  |  |  |  |  |  |  |  | $ 19.085 | $ 19.085 |  | $ 19.085 |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 5,700,000 |  | $ 200,000 |  |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 4,328 | 4,328 |  | 4,500 |  | 4,328 |  |  |  |  |  |
Shares issued | shares | 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercisable price | (per share) | $ 19.085 |  |  |  |  |  |  |  | $ 19.085 | $ 19.085 |  | $ 0 |  |  |  |  |  |  |  |
IISI Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of common share each option is eligible | shares | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period | seven years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period | two years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise Price |  |  |  |  |  |  |  |  | $ 14.00 | $ 14.00 |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |
Outstanding stock options |  |  |  |  |  |  |  |  | 530.00 | 530.00 |  |  |  | 530.00 |  |  |  |  |  |
Shares issued | shares |  |  |  |  |  | 1,335,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercisable price | (per share) |  |  |  |  |  |  |  |  | $ 14.00 | $ 14.00 |  |  |  | $ 14 |  |  |  |  |  |
Shares options | shares |  |  |  |  | 1,665,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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