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Inventories - Additional Information (Detail) $ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2017 TWD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 TWD ($) | Dec. 31, 2015 TWD ($)
Classes Of Inventories [Abstract] |  |  |  |
Operating costs related to inventories | $ 56,342 |  | $ 54,183 | $ 52,666
Provision for inventory and obsolescence recognized as operating costs | 52 | $ 2 | 192 | $ 198
Inventory to be recovered longer than twelve months | $ 2,076 |  | $ 2,074 |

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