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Retirement Benefit Plans - Summary of Life Expectancies for Employee Retiring at Age 65 (Detail)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of defined benefit plans [abstract]    
Retiring at the end of the reporting period, Male 85.2 years 85.2 years
Retiring 20 years after the end of the reporting period, Male 86.3 years 86.3 years
Retiring at the end of the reporting period, Female 87.7 years 87.6 years
Retiring 20 years after the end of the reporting period, Female 88.6 years 88.6 years