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Equity - Summary of Shares Held By Subsidiaries (Detail) shares in Thousands, $ in Thousands, $ in Thousands | Dec. 31, 2018 USD ($) shares | Dec. 31, 2018 TWD ($) shares | Dec. 31, 2017 TWD ($) shares
--- | --- | --- | ---
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 72,941 | 72,941 | 145,883
Shares Held By Subsidiaries, Value | $ 64,002 | $ 1,959,107 | $ 1,959,107
ASE Test Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 44,100 | 44,100 | 88,200
Shares Held By Subsidiaries, Value | $ 45,107 | $ 1,380,721 | $ 1,380,721
J & R Holding Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 23,352 | 23,352 | 46,704
Shares Held By Subsidiaries, Value | $ 12,470 | $ 381,709 | $ 381,709
ASE Test, Inc. [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 5,489 | 5,489 | 10,979
Shares Held By Subsidiaries, Value | $ 6,425 | $ 196,677 | $ 196,677
Fair value [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | 138,925 | 4,252,490 | 5,565,437
Fair value [member] | ASE Test Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | 83,994 | 2,571,044 | 3,364,848
Fair value [member] | J & R Holding Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | 44,476 | 1,361,415 | 1,781,749
Fair value [member] | ASE Test, Inc. [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | $ 10,455 | $ 320,031 | $ 418,840

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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