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Significant Contingent Liabilities and Unrecognized Commitments - Additional Information (Detail) - TWD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017 | Aug. 15, 1996
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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 | $ 98 |  |
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 |  | 118 |
Telecommunications equipment [Member] |  |  |
Commitments And Contingencies [Line Items] |  |  |
Commitments for acquisitions of property, plant and equipment |  | $ 16,199 |

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- Definition The amount of contractual commitments for the acquisition of property, plant and equipment. [Refer: Property, plant and equipment] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 16 -IssueDate 2016-01-13 -Paragraph 74 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=16&date=2016-03-30&anchor=para_74_c&doctype=Standard -URIDate 2016-03-31 + Details
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