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Income Tax - Summary of Projects were Exempted From Income Tax (Detail)
 12 Months Ended
Dec. 31, 2019
Construction and expansion of 2007 by the company [member]  
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]  
Tax-exemption period, start date 2016-01
Tax-exemption period, end date 2020-12
Construction and expansion of 2009 [member] | ASE Test, Inc. [member]  
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]  
Tax-exemption period, start date 2018-01
Tax-exemption period, end date 2022-12
Construction and expansion of 2008 [member] | ASE Electronics Inc. [member]  
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]  
Tax-exemption period, start date 2016-01
Tax-exemption period, end date 2020-12