Retirement Benefit Plans - Summary of Weighted Average Assumptions Used to Calculate the Defined Benefit Obligation (Detail)	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Pension Plans
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Discount rate	2.39%	3.04%
Rate of increase in future compensation	3.25%	3.25%
Non-Pension Post- Retirement Benefit Plans
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Discount rate	2.50%	3.10%
Rate of increase in future compensation	3.25%	3.25%
Medical trend rate	5.00%	5.00%

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2020-01-01
-Paragraph 144
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-URIDate 2020-01-01

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- Definition

The expected rate of salary increases used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]]

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Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

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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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