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Equity - Summary of Shares Held By Subsidiaries (Detail) shares in Thousands, $ in Thousands, $ in Thousands | Dec. 31, 2019 TWD ($) shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 TWD ($) shares
--- | --- | --- | ---
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 72,941 | 72,941 | 72,941
Shares Held By Subsidiaries, Value | $ 1,959,107 | $ 65,500 | $ 1,959,107
ASE Test Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 44,100 | 44,100 | 44,100
Shares Held By Subsidiaries, Value | $ 1,380,721 | $ 46,162 | $ 1,380,721
J & R Holding Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 23,352 | 23,352 | 23,352
Shares Held By Subsidiaries, Value | $ 381,709 | $ 12,762 | $ 381,709
ASE Test, Inc. [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 5,489 | 5,489 | 5,489
Shares Held By Subsidiaries, Value | $ 196,677 | $ 6,576 | $ 196,677
Fair value [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | 6,068,733 | 202,900 | 4,252,490
Fair value [member] | ASE Test Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | 3,669,140 | 122,673 | 2,571,044
Fair value [member] | J & R Holding Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | 1,942,876 | 64,957 | 1,361,415
Fair value [member] | ASE Test, Inc. [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | $ 456,717 | $ 15,270 | $ 320,031

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