Hedging Financial Instruments - Summary of Hedging Transaction and Reclassification Affected Comprehensive Income (Details) - Forecast Purchases - forward Exchange Contract [Member] - Cash flow hedges [member] - TWD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Reclassification Of Financial Assets [Line Items]
Hedging Gain or Loss Recognized in OCI	$ (1)	$ 2
Amount of Hedge Ineffectiveness Recognized in Profit or Loss	0	0
Line item in which hedge ineffectiveness is included	0	0
Amount Reclassified to P/L and the Adjusted Line Item	(2)	(4)
Due to Hedged Future Cash Flows No Longer Expected to Occur	$ 0	$ 0