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Significant Contingent Liabilities and Unrecognized Commitments - Additional Information (Detail) $ in Thousands, $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2013 TWD ($) | Jan. 31, 2018 TWD ($) | Jan. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Disclosure of contingent liabilities [line items] |  |  |  |  |  |  |
Unused letters of credit | $ 20,000 |  | $ 97,000 |  |  |  | $ 675
Committed to purchase property, plant and equipment | 7,019,377 |  | 6,630,957 |  |  |  | $ 236,821
Prepayment of property, plant and equipment | 294,194 | $ 9,926 | 668,509 |  |  |  |
Construction contract related to real estate business | $ 1,548,806 | $ 52,254 | $ 1,574,822 |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |
Disclosure of contingent liabilities [line items] |  |  |  |  |  |  |
Contribution to be made for environmental protection |  |  |  |  | $ 100,000 | $ 3,374 |
Taiwan [member] |  |  |  |  |  |  |
Disclosure of contingent liabilities [line items] |  |  |  |  |  |  |
Contractual period for environmental protection |  |  |  | 30 years |  |  |
Contribution to be made for environmental protection |  |  |  | $ 3,000,000 |  |  |

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