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Equity - Summary of Shares Held By Subsidiaries (Detail) shares in Thousands, $ in Thousands, $ in Thousands | Dec. 31, 2017 TWD ($) shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 TWD ($) shares
--- | --- | --- | ---
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 145,883 | 145,883 | 145,883
Shares Held By Subsidiaries, Value | $ 1,959,107 | $ 66,097 | $ 1,959,107
Fair value [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | $ 5,565,437 | $ 187,768 | $ 4,821,434
ASE Test Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 88,200 | 88,200 | 88,200
Shares Held By Subsidiaries, Value | $ 1,380,721 | $ 46,583 | $ 1,380,721
ASE Test Limited [member] | Fair value [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | $ 3,364,848 | $ 113,524 | $ 2,915,026
J & R Holding Limited [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 46,704 | 46,704 | 46,704
Shares Held By Subsidiaries, Value | $ 381,709 | $ 12,878 | $ 381,709
J & R Holding Limited [member] | Fair value [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | $ 1,781,749 | $ 60,113 | $ 1,543,559
ASE Test, Inc. [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries (in thousands) | 10,979 | 10,979 | 10,979
Shares Held By Subsidiaries, Value | $ 196,677 | $ 6,636 | $ 196,677
ASE Test, Inc. [member] | Fair value [member] |  |  |
Disclosure of subsidiaries [Line Items] |  |  |
Shares Held By Subsidiaries, Value | $ 418,840 | $ 14,131 | $ 362,849

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