Capital stock - Movement of Treasury Stock (Detail) shares in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2018 TWD ($) shares
Capital reduction	$ 1,284,223
Treasury stock [member]
Treasury stock shares at beginning balance \| shares	30,085
Capital reduction \| shares	(4,515)
Treasury stock shares at ending balance \| shares	25,570
Beginning balance	$ 1,007,654
Capital reduction	(45,151)
Ending balance	$ 962,503

An entity’s own equity instruments, held by the entity or other members of the consolidated group.

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