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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
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Apr. 29, 2018 TWD ($) shares $ / shares | Dec. 31, 2018 shares $ / shares | Dec. 31, 2019 shares $ / shares | Dec. 31, 2018 shares $ / shares | Dec. 31, 2017 shares $ / shares
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |
Weighted average exercise price, Beginning balance | $ / shares |  |  | $ 58.1 |  |
Options granted | $ / shares |  | $ 56.4 |  |  |
Weighted average exercise price, Options forfeited | $ / shares |  | 71.5 |  |  |
Weighted average exercise price, Ending balance | $ / shares |  | $ 58.1 |  | $ 58.1 |
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
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)
Number of options, expired | shares |  |  |  |  | (1,790,000)
Number of options, exercised | shares | (20,557,000) |  |  |  | (67,637,000)
Number of options, Ending balance | shares |  |  |  |  | 135,961,000
Number of options, exercisable, end of year | shares |  |  |  |  | 85,642,000
Weighted average exercise price, Beginning balance | $ / shares | $ 30.2 |  |  | $ 30.2 | $ 27.3
Transferred from ASE | $ | $ 30.9 |  |  |  |
Weighted average exercise price, Options forfeited | $ / shares | $ 36.3 |  |  |  | 36.3
Weighted average exercise price, Options expired | $ / shares |  |  |  |  | 21.1
Weighted average exercise price, Options exercised | $ / shares | $ 26.0 |  |  |  | 21.0
Weighted average exercise price, Ending balance | $ / shares |  |  |  |  | 30.2
Weighted average exercise price, Options exercisable, end of year | $ / shares |  |  |  |  | $ 26.5
The Company Option Plans [member] |  |  |  |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |  |  |  |
Number of options, beginning balance | shares |  | 56,856,000 | 183,814,000 |  |
Options granted | shares |  | 131,863,000 |  |  |
Number of options, forfeited | shares |  | (1,582,000) | (4,214,000) |  |
Number of options, exercised | shares |  | (3,323,000) | (8,814,000) |  |
Number of options, Ending balance | shares |  | 183,814,000 | 170,786,000 | 183,814,000 |
Number of options, exercisable, end of year | shares |  | 36,354,000 | 33,822,000 | 36,354,000 |
Weighted average exercise price, Beginning balance | $ / shares |  | $ 61.7 | $ 58.1 |  |
Weighted average exercise price, Options forfeited | $ / shares |  |  | 61.8 |  |
Weighted average exercise price, Options exercised | $ / shares |  | 43.6 | 48.4 |  |
Weighted average exercise price, Ending balance | $ / shares |  | 58.1 | 57.0 | $ 58.1 |
Weighted average exercise price, Options exercisable, end of year | $ / shares |  | $ 58.1 | $ 63.5 | $ 58.1 |
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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