Segment Information - Geographic Information (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 geographical areas [Line Items]
Revenues	$ 207,609	$ 7,393	$ 207,520	$ 215,483
Taiwan, ROC [Member]
Disclosure of geographical areas [Line Items]
Revenues	200,881		197,895	205,696
Overseas [Member]
Disclosure of geographical areas [Line Items]
Revenues	$ 6,728		$ 9,625	$ 9,787

The income arising in the course of an entity's ordinary activities. Income is increases in assets, or decreases in liabilities, that result in increases in equity, other than those relating to contributions from holders of equity claims.

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+ Details