Contents of Significant Accounts - Post-Employment Benefits - Additional Information (Detail) - TWD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of defined benefit plans and defined contribution plans [Line Items]
Actual returns on plan assets	$ 61	$ 55
Expected pension fund contribution in 2021	$ 517
Weighted-average durations of defined benefit obligation	9 years	9 years
Defined benefit plan [member]
Disclosure of defined benefit plans and defined contribution plans [Line Items]
Percentages of employees' total monthly salaries and wages to pension fund	2.00%	2.00%	2.00%
Pension expenses	$ 40	$ 59	$ 69
Defined contribution plan [member]
Disclosure of defined benefit plans and defined contribution plans [Line Items]
Minimum percentage of employees' monthly salary or wages as monthly contributions to defined contribution plan	6.00%	6.00%	6.00%
Pension expenses	$ 1,478	$ 1,369	$ 1,339

The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The amount of post-employment benefit expense relating to defined benefit plans. [Refer: Defined benefit plans [member]]

+ References

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The amount of post-employment benefit expense relating to defined contribution plans. Defined contribution plans are post-employment benefit plans under which an entity pays fixed contributions into a separate entity (a fund) and will have no legal or constructive obligation to pay further contributions if the fund does not hold sufficient assets to pay all employee benefits relating to employee service in the current and prior periods.

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The weighted average duration of a defined benefit obligation. [Refer: Weighted average [member]]

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Actual return on plan assets.

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Disclosure of defined benefit plans and defined contribution plans [Line Items]

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Minimum percentage of monthly salary or wages as monthly contributions to defined contribution plan.

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Percentage of employees' monthly salaries and wages as contributions to defined benefit plan.

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