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Intangible Assets - Additional Information (Detail) - TWD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Feb. 29, 2020
Oct. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about intangible assets [Line Items]          
5G spectrum bidding amount   $ 1,000      
Additions-acquired separately $ 48,373        
Senao Networks, Inc. ("SNI") [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Discount rate     12.10% 12.30% 13.70%
Senao Networks, Inc. ("SNI") [Member] | Goodwill [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Impairment loss     $ 9 $ 9 $ 0
Senao Networks, Inc. ("SNI") [Member] | Certain licensed contract [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Impairment loss         $ 51
Bottom of range [Member] | Computer software [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Estimated useful lives     1 year    
Bottom of range [Member] | Others [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Estimated useful lives     1 year    
Top of Range [Member] | Computer software [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Estimated useful lives     10 years    
Top of Range [Member] | Others [Member]          
Disclosure of detailed information about intangible assets [Line Items]          
Estimated useful lives     20 years