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Share-based Payment Arrangements - Summary of ASE Inc. Option Plans Outstanding Share Options (Detail) | Dec. 31, 2018 TWD ($) yr | Dec. 31, 2017 TWD ($) yr
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ASE Inc. option plans [member] | Fourth share options exercise price [member] |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |
Weighted Average Remaining Contractual Life (Years) | yr | 1.5 | 2.5
ASE Inc. option plans [member] | Fifth share options exercise price [member] |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |
Range of Exercise Price Per Share | $ 73.0 | $ 36.5
Weighted Average Remaining Contractual Life (Years) | yr | 6.7 | 7.7
ASE Inc. option plans [member] | Bottom of range [member] | Fourth share options exercise price [member] |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |
Range of Exercise Price Per Share | $ 40.8 | $ 20.4
ASE Inc. option plans [member] | Top of range [member] | Fourth share options exercise price [member] |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |
Range of Exercise Price Per Share | 45.2 | $ 22.6
The Company Option Plans [member] | First share options exercise price [member] |  |
Disclosure of terms and conditions of share-based payment arrangement [Line Items] |  |
Range of Exercise Price Per Share | $ 56.4 |
Weighted Average Remaining Contractual Life (Years) | yr | 9.9 |

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