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Retirement Benefit Plans - Effect on Present Value of Defined Benefit Obligation Due to a Reasonably Possible Change of the Significant Actuarial Assumptions (Detail) - TWD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Discount rates [Member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
0.5% increase	$ (1,208)	$ (1,275)
0.5% decrease	1,284	1,356
Expected rates of salary increases [Member]
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]
0.5% increase	1,372	1,448
0.5% decrease	$ (1,303)	$ (1,374)

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- Definition

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+ References

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- Definition

The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]]

+ References

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- Definition

The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]]

+ References

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