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Investment Properties - Summary of Investment Properties (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
TWD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
TWD ($)
Dec. 31, 2018
TWD ($)
Disclosure of detailed information about investment property [Line Items]        
Beginning balance $ 8,169      
Depreciation expense (22)   $ (25) $ (21)
Reversal of impairment loss 27 $ 1 57 19
Ending balance 9,621 $ 343 8,169  
Carrying Amount [Member]        
Disclosure of detailed information about investment property [Line Items]        
Beginning balance 9,214   9,392 9,135
Additions 1,359   1 6
Disposal (37)   (6)  
Reclassification 126   (173) 251
Ending balance 10,662   9,214 9,392
Accumulated depreciation, amortization and impairment [Member]        
Disclosure of detailed information about investment property [Line Items]        
Beginning balance (1,045)   (1,105) (1,087)
Depreciation expense (22)   (25) (21)
Disposal     6  
Reclassification (1)   22 (16)
Reversal of impairment loss 27   57 19
Ending balance $ (1,041)   $ (1,045) $ (1,105)