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Retirement Benefit Plans - Principal Assumptions Used for Actuarial Valuations (Detail)
Dec. 31, 2020
Dec. 31, 2019
Disclosure Of Defined Plan [line items]    
Discount rates 0.50% 0.75%
Bottom of range [Member]    
Disclosure Of Defined Plan [line items]    
Expected rates of salary increase 1.00% 1.20%
Top of range [Member]    
Disclosure Of Defined Plan [line items]    
Expected rates of salary increase 2.00% 2.00%