Segment Information - Geographic Information (Detail) $ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018 TWD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 TWD ($)	Dec. 31, 2016 TWD ($)
Disclosure of geographical areas [Line Items]
Revenues	$ 215,483	$ 7,040	$ 227,514	$ 229,991
Taiwan, ROC [Member]
Disclosure of geographical areas [Line Items]
Revenues	205,696		217,568	218,933
Overseas [Member]
Disclosure of geographical areas [Line Items]
Revenues	$ 9,787		$ 9,946	$ 11,058

The income arising in the course of an entity's ordinary activities. Income is increases in economic benefits during the accounting period in the form of inflows or enhancements of assets or decreases of liabilities that result in an increase in equity, other than those relating to contributions from equity participants.

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+ Details