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Significant Contingent Liabilities and Unrecognized Commitments - Additional Information (Detail) - TWD ($) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2020 | Dec. 31, 2019 | Jul. 31, 2017 | Aug. 15, 1996
Commitments And Contingencies [Line Items] |  |  |  |
Unused letters of credit |  | $ 50 |  |
Commitment amount |  |  | $ 1,614 |
Contractual Consideration Payment [Member] |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |
Contractual consideration payment | $ 1,533 |  |  |
Top of Range [Member] | Next Commercial Bank Co Ltd [Member] |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |
Ownership interest in associates |  | 25.00% |  |
Piping Fund [Member] |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |
Commitment amount |  |  |  | $ 2,000
Commitment to contribute to a Piping Fund, amount contributed |  |  |  | $ 1,000
Commitment to contribute to a Piping Fund, remaining amount |  | $ 1,000 |  |
Land and buildings [Member] |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |
Commitments for acquisitions of property, plant and equipment |  | 44 |  |
Telecommunications equipment [Member] |  |  |  |
Commitments And Contingencies [Line Items] |  |  |  |
Commitments for acquisitions of property, plant and equipment |  | $ 17,877 |  |

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