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Bonds Payable - Summary of Major Terms of Unsecured Domestic Bonds Issued (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Disclosure Of Detailed Information About Bonds Payable [Line Items]  
Total Amount $ 20,000
Unsecured Domestic Bonds [Member] | Tranche A [Member]  
Disclosure Of Detailed Information About Bonds Payable [Line Items]  
Issuance 2020-1
Issuance Period July 2020 to July 2025
Total Amount $ 8,800
Interest rates of loans 0.50%
Repayment and Interest Payment One-time repayment upon maturity; interest payable annually
Unsecured Domestic Bonds [Member] | Tranche B [Member]  
Disclosure Of Detailed Information About Bonds Payable [Line Items]  
Issuance Period July 2020 to July 2027
Total Amount $ 7,500
Interest rates of loans 0.54%
Repayment and Interest Payment The same as above
Unsecured Domestic Bonds [Member] | Tranche C [Member]  
Disclosure Of Detailed Information About Bonds Payable [Line Items]  
Issuance Period July 2020 to July 2030
Total Amount $ 3,700
Interest rates of loans 0.59%
Repayment and Interest Payment The same as above