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Share-based Payments Arrangement - Additional Information (Detail) $ / shares in Units, $ / shares in Units, $ in Millions |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |
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Dec. 20, 2019 $ / shares shares | Oct. 31, 2018 shares | Dec. 19, 2017 $ / shares shares | Feb. 08, 2017 shares | Oct. 22, 2015 shares | May 07, 2013 shares | Mar. 31, 2018 shares | Dec. 31, 2019 TWD ($) $ / shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 TWD ($) $ / shares shares | Dec. 31, 2017 TWD ($) $ / shares shares | Jun. 30, 2019 $ / shares | Aug. 31, 2018 $ / shares | Jul. 31, 2018 $ / shares | Jun. 30, 2018 $ / shares | Jul. 31, 2017 $ / shares | Jun. 30, 2017 $ / shares | Dec. 31, 2016
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs |  |  |  |  |  |  |  | $ 2,000,000 | $ 0 | $ 17,000,000 | $ 22,000,000 |  |  |  |  |  |  |
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 |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |
Option expire period |  |  |  |  |  | six years |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  | $ 0 |  | $ 0 | $ 4,000,000 |  |  |  |  |  |  |
Exercise Price | $ / shares |  |  |  |  |  |  |  | $ 66.20 |  | $ 66.20 |  |  |  | $ 66.20 | $ 70.70 | $ 70.70 | $ 76.10 |
Number of Options Outstanding |  |  |  |  |  |  |  | 0 |  | 5,318,000 | 5,926,000 |  |  |  |  |  |  | 6,587,000
Exercisable price | $ / shares |  |  |  |  |  |  |  | $ 0 |  | $ 66.20 | $ 70.70 |  |  |  |  |  |  |
Senao International Co., Ltd. (SENAO) [Member] | Stock Option Granted in May and November 2017 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  |  | $ 9,000,000 |  |  |  |  |  |  |
Number of treasury stock transfer to its specific employees |  |  |  |  |  |  |  |  |  |  | 3,342,000 |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  | 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 |  | 1,000 | 1,000 |  | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |
Shares issued |  |  |  |  |  |  | 7,842,000 |  |  |  |  |  |  |  |  |  |  |
Reserve for employee subscription |  |  |  |  |  |  | 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 |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  | $ 300,000 |  | $ 1,000,000 | $ 3,000,000 |  |  |  |  |  |  |
Exercise Price | $ / shares |  |  |  |  |  |  |  | $ 34.40 |  | $ 34.40 |  |  |  |  |  |  |  |
Number of Options Outstanding |  |  |  |  |  |  |  | 314.25 |  | 882.75 | 1,936.00 |  |  |  |  |  |  | 1,948.00
Exercisable price | $ / shares |  |  |  |  |  |  |  | $ 34.40 |  | $ 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 |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  | $ 600,000 |  | $ 600,000 | $ 100,000 |  |  |  |  |  |  |
Exercise Price | $ / shares |  |  | $ 147.00 |  |  |  |  | $ 135.60 |  | $ 140.60 |  | $ 135.60 | $ 140.60 | $ 144.10 | $ 144.10 |  |  |
Number of Options Outstanding |  |  |  |  |  |  |  | 897.00 |  | 925.00 | 950.00 |  |  |  |  |  |  | 0
Exercisable price | $ / shares |  |  |  |  |  |  |  | $ 135.60 |  | $ 0 | $ 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 |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  | $ 600,000 |  | $ 100,000 |  |  |  |  |  |  |  |
Exercise Price | $ / shares |  |  |  |  |  |  |  | $ 141.70 |  | $ 147.00 |  | $ 141.70 |  |  |  |  |  |
Number of Options Outstanding |  |  |  |  |  |  |  | 46.00 |  | 50.00 | 0 |  |  |  |  |  |  |
Exercisable price | $ / shares |  |  |  |  |  |  |  | $ 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 | 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option expire period | five years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period | one year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |
Exercise Price | $ / shares |  |  |  |  |  |  |  | $ 19.085 |  |  |  |  |  |  |  |  |  |
Number of Options Outstanding |  |  |  |  |  |  |  | 4,500 |  |  |  |  |  |  |  |  |  |
Shares issued | 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercisable price | (per share) | $ 19.085 |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |
Chunghwa Precision Test Tech. Co., Ltd. (CHPT) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compensation costs | $ |  |  |  |  |  |  |  |  |  |  | $ 6,000,000 |  |  |  |  |  |  |
Shares issued |  |  |  | 2,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reserve for employee subscription |  |  |  | 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |

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