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Defined Benefit Plans - Summary of Actuarial Assumptions Used to Determine Defined Benefit Obligations (Parenthetical) (Detail) - Counterparty
Dec. 31, 2019
Dec. 31, 2017
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
White collar jobs percentage 90.00%  
Blue collar jobs percentage 10.00%  
Top of range [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Club Vita tables Scheme membership CMI   0.0150
Bottom of range [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Club Vita tables Scheme membership CMI   0.0125
Remeasurement of defined benefit Plans of group companies [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Discount rate 3.23%  
Post retirement welfare plan [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Discount rate 3.02%