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Equity - Summary of Common Stocks (Detail) $ in Millions | Dec. 31, 2017 TWD ($) shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 TWD ($) shares
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Disclosure Of Classes Of Share Capital [Abstract] |  |  |
Number of authorized shares | 12,000,000,000 | 12,000,000,000 | 12,000,000,000
Authorized shares | $ | $ 120,000,000,000 |  | $ 120,000,000,000
Number of issued and paid shares | 7,757,000,000 | 7,757,000,000 | 7,757,000,000
Issued and outstanding shares | $ 77,574,000,000 | $ 2,617 | $ 77,574,000,000

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