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Revenue - Summary of Revenue Recognized (Detail) - TWD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue [Abstract]      
Telecommunications business $ 5,492 $ 6,186 $ 7,157
Project business 6,092 3,973 627
Others 512 404 324
Contracts with customers revenue recognized $ 12,096 $ 10,563 $ 8,108