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Net Income - Other Income (Detail) $ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Analysis Of Income And Expense [Abstract] |  |  |  |
Dividend income | $ 246 | $ 9 | $ 296 | $ 396
Rental income | 70 |  | 85 | 70
Others | 154 |  | 150 | 234
Other income | $ 470 | $ 17 | $ 531 | $ 700

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