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Share-based Payment Arrangements - Summary of Information About ASE Inc. Option Plans (Detail) | 4 Months Ended | 8 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 29, 2018 TWD ($) shares | Dec. 31, 2018 TWD ($) shares | Dec. 31, 2018 TWD ($) shares | Dec. 31, 2017 TWD ($) shares | Dec. 31, 2016 TWD ($) shares
ASE Inc. option plans [member] |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |
Number of options, beginning balance | shares | 135,961,000 |  | 135,961,000 | 210,795,000 | 252,607,000
Number of options, transferred to the Company in accordance with the joint share exchange agreement | shares | 113,712,000 |  |  |  |
Number of options, forfeited | shares | (1,692,000) |  |  | (5,407,000) | (6,056,000)
Number of options, expired | shares |  |  |  | (1,790,000) |
Number of options, exercised | shares | (20,557,000) |  |  | (67,637,000) | (35,756,000)
Number of options, Ending balance | shares |  |  |  | 135,961,000 | 210,795,000
Number of options, exercisable, end of period | shares |  |  |  | 85,642,000 | 123,007,000
Weighted average exercise price, Beginning balance | $ 30.2 |  | $ 30.2 | $ 27.3 | $ 26.6
Transferred from ASE | 30.9 |  |  |  |
Weighted average exercise price, Options forfeited | 36.3 |  |  | 36.3 | 34.6
Weighted average exercise price, Options expired |  |  |  | 21.1 |
Weighted average exercise price, Options exercised | $ 26.0 |  |  | 21.0 | 20.9
Weighted average exercise price, Ending balance |  |  |  | 30.2 | 27.3
Weighted average exercise price, Options exercisable, end of period |  |  |  | $ 26.5 | $ 20.8
The Company Option Plans [member] |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |
Number of options, transferred to the Company in accordance with the joint share exchange agreement |  | 56,856 |  |  |
Options granted |  | 131,863 |  |  |
Number of options, forfeited |  | (1,582) |  |  |
Number of options, exercised |  | (3,323) |  |  |
Number of options, Ending balance |  | 183,814 | 183,814 |  |
Number of options, exercisable, end of period |  | 36,354 | 36,354 |  |
Transferred from ASE |  | $ 61.7 |  |  |
Options granted |  | 56.4 |  |  |
Weighted average exercise price, Options forfeited |  | 71.5 |  |  |
Weighted average exercise price, Options exercised |  | 43.6 |  |  |
Weighted average exercise price, Ending balance |  | 58.1 | $ 58.1 |  |
Weighted average exercise price, Options exercisable, end of period |  | 58.1 | 58.1 |  |
Fair value of options granted (NT$) |  | $ 0 | $ 0 |  |
The Company Option Plans [member] | Bottom of range [member] |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |
Fair value of options granted (NT$) | shares |  | 16.28 | 16.28 |  |
The Company Option Plans [member] | Top of range [member] |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |
Fair value of options granted (NT$) | shares |  | 19.12 | 19.12 |  |

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