Segment Information - Geographic Information (Detail) $ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 TWD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 TWD ($)	Dec. 31, 2015 TWD ($)
Disclosure of geographical areas [Line Items]
Revenues	$ 227,514	$ 7,676	$ 229,991	$ 231,795
Taiwan, ROC [Member]
Disclosure of geographical areas [Line Items]
Revenues	217,568		218,933	220,917
Overseas [Member]
Disclosure of geographical areas [Line Items]
Revenues	$ 9,946		$ 11,058	$ 10,878

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