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Retirement Benefit Plans - Movements in Present Value of Defined Benefit Obligation (Detail) - TWD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Remeasurement:
Actuarial loss (gain) arising from experience adjustments	$ 494.1	$ (438.0)	$ 334.7
Actuarial gain arising from changes in demographic assumptions		(233.2)
Actuarial loss arising from changes in financial assumptions	3,161.9	541.7	597.8
Present value of defined benefit obligation [member]
Disclosure of defined benefit plans [line items]
Balance, beginning of year	13,484.1	13,662.7	12,774.6
Current service cost	123.3	135.6	137.7
Interest expense	118.8	175.4	207.8
Remeasurement:
Actuarial loss (gain) arising from experience adjustments	494.1	(438.0)	334.7
Actuarial gain arising from changes in demographic assumptions		(233.2)
Actuarial loss arising from changes in financial assumptions	3,161.9	541.7	597.8
Benefits paid from plan assets	(399.0)	(344.1)	(274.3)
Benefits paid directly by the Company	(2.9)	(16.0)	(115.6)
Balance, end of year	$ 16,980.3	$ 13,484.1	$ 13,662.7

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

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- Definition

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

+ References

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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 domain members defined in one or many axes of the table.

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No definition available.

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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)]

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- Definition

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]]

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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)]

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