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Lease Arrangements - Summary of Total Cash Outflow for Leases (Details) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Quantitative Information About Leases For Lessee [Abstract]    
Expenses relating to low-value asset leases $ 8 $ 7
Expenses relating to variable lease payments not included in the measurement of lease liabilities 5 6
Total cash outflow for leases $ 3,776 $ 3,826