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Pensions - Movements in Net Defined Benefit Liability (Detail) - TWD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	$ (480,107)	$ (520,765)	$ (478,526)
Current service cost	(263)	(332)	(382)
Interest (expense) income	(4,664)	(6,339)	(8,138)
Total	(485,034)	(527,436)	(487,046)
Return of plan assets (excluding the amount included in interest income or expense)	12,568	12,601	8,145
Financial assumption movement effect	(57,180)	(27,993)	(56,934)
Experience adjustments	(7,378)	36,308	(11,172)
Gain (loss) on remeasurement, net defined benefit liability (asset)	(51,990)	20,916	(59,961)
Pension fund contribution	25,373	26,413	26,242
Ending balance	(511,651)	(480,107)	(520,765)
Present Value of Defined Benefit Obligation [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	(901,159)	(910,081)	(838,543)
Current service cost	(263)	(332)	(382)
Interest (expense) income	(8,835)	(11,170)	(14,429)
Total	(910,257)	(921,583)	(853,354)
Financial assumption movement effect	(57,180)	(27,993)	(56,934)
Experience adjustments	(7,378)	36,308	(11,172)
Gain (loss) on remeasurement, net defined benefit liability (asset)	(64,558)	8,315	(68,106)
Paid pension	31,424	12,109	11,379
Ending balance	(943,391)	(901,159)	(910,081)
Fair Value of Plan Assets [member]
Disclosure of net defined benefit liability (asset) [Line Items]
Beginning balance	421,052	389,316	360,017
Interest (expense) income	4,171	4,831	6,291
Total	425,223	394,147	366,308
Return of plan assets (excluding the amount included in interest income or expense)	12,568	12,601	8,145
Gain (loss) on remeasurement, net defined benefit liability (asset)	12,568	12,601	8,145
Pension fund contribution	25,373	26,413	26,242
Paid pension	(31,424)	(12,109)	(11,379)
Ending balance	$ 431,740	$ 421,052	$ 389,316

X

- 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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- Definition

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

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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

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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 domain 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)]

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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

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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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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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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 plans 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]; Interest expense (income), net defined benefit liability (asset)]

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

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