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Segment Information - Net Revenue from Customers Representing at Least 10% of Total Revenue (Detail) $ in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2020 TWD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 TWD ($) | Dec. 31, 2018 TWD ($)
Disclosure of major customers [Line Items] |  |  |  |
Net revenue | $ 23,011,381 | $ 819,493 | $ 20,337,881 | $ 18,480,027
Percentage of total revenue | 100.00% | 100.00% | 100.00% | 100.00%
Customer A [member] |  |  |  |
Disclosure of major customers [Line Items] |  |  |  |
Net revenue | $ 5,088,544 |  | $ 4,756,755 | $ 3,794,991
Percentage of total revenue | 22.00% | 22.00% | 23.00% | 21.00%
Customer X [member] |  |  |  |
Disclosure of major customers [Line Items] |  |  |  |
Net revenue | $ 2,660,866 |  | $ 1,977,427 | $ 1,328,752
Percentage of total revenue | 12.00% | 12.00% | 10.00% | 7.00%
Customer K [member] |  |  |  |
Disclosure of major customers [Line Items] |  |  |  |
Net revenue | $ 2,332,914 |  | $ 2,419,612 | $ 2,637,053
Percentage of total revenue | 10.00% | 10.00% | 12.00% | 14.00%
Customer C [member] |  |  |  |
Disclosure of major customers [Line Items] |  |  |  |
Net revenue | $ 2,143,130 |  | $ 2,048,260 | $ 1,957,467
Percentage of total revenue | 9.00% | 9.00% | 10.00% | 11.00%

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