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Financial Instruments - Information About the Maturity Analysis for Lease Liabilities (Detail) - TWD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of maturity analysis for non-derivative financial liabilities [Line Items]    
Lease liabilities $ 9,737 $ 9,930
Less Than 1 Year [Member]    
Disclosure of maturity analysis for non-derivative financial liabilities [Line Items]    
Lease liabilities 3,397 3,310
1-3 Years [Member]    
Disclosure of maturity analysis for non-derivative financial liabilities [Line Items]    
Lease liabilities 4,240 4,394
3-5 Years [Member]    
Disclosure of maturity analysis for non-derivative financial liabilities [Line Items]    
Lease liabilities 1,691 1,581
More Than 5 Years [Member]    
Disclosure of maturity analysis for non-derivative financial liabilities [Line Items]    
Lease liabilities $ 409 $ 645