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Pensions - Movements in Net Defined Benefit Liability (Detail) - TWD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	$ (511,651)	$ (480,107)	$ (520,765)
Current service cost	(237)	(263)	(332)
Interest (expense) income	(2,492)	(4,664)	(6,339)
Total	(514,380)	(485,034)	(527,436)
Return on plan assets (excluding amounts included in interest income or expense)	5,613	12,568	12,601
Impact on changes in demographic assumptions	(20,022)
Financial assumption movement effect	23,757	(57,180)	(27,993)
Experience adjustments	(24,347)	(7,378)	36,308
Gain (loss) on remeasurement, net defined benefit liability (asset)	(14,999)	(51,990)	20,916
Pension fund contribution	26,091	25,373	26,413
Ending balance	(503,288)	(511,651)	(480,107)
Present Value of Defined Benefit Obligation [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(943,391)	(901,159)	(910,081)
Current service cost	(237)	(263)	(332)
Interest (expense) income	(4,629)	(8,835)	(11,170)
Total	(948,257)	(910,257)	(921,583)
Impact on changes in demographic assumptions	(20,022)
Financial assumption movement effect	23,757	(57,180)	(27,993)
Experience adjustments	(24,347)	(7,378)	36,308
Gain (loss) on remeasurement, net defined benefit liability (asset)	(20,612)	(64,558)	8,315
Paid pension	9,192	31,424	12,109
Ending balance	(959,677)	(943,391)	(901,159)
Fair Value of Plan Assets [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	431,740	421,052	389,316
Interest (expense) income	2,137	4,171	4,831
Total	433,877	425,223	394,147
Return on plan assets (excluding amounts included in interest income or expense)	5,613	12,568	12,601
Gain (loss) on remeasurement, net defined benefit liability (asset)	5,613	12,568	12,601
Pension fund contribution	26,091	25,373	26,413
Paid pension	(9,192)	(31,424)	(12,109)
Ending balance	$ 456,389	$ 431,740	$ 421,052

X

- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

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The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

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-Name IAS
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plan assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Increase (decrease) in net defined benefit liability (asset) resulting from interest expense (income)]

+ References

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Current service cost, and Interest (expense) income, net defined benefit liability (asset).

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No definition available.

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