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Pension and Other Employee Benefits - Summary of Weighted Average Assumptions Used to Determine Pension Benefits of Defined Benefit Pension Plans (Detail)
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Defined Benefit Plans [Line Items]      
Rate of increase in compensation 6.00% 6.00% 6.00%
Discount rate 3.50% 4.80% 7.30%
Investment Property [Member] | Smart and Subsidiaries [Member]      
Disclosure Of Defined Benefit Plans [Line Items]      
Rate of increase in compensation 5.00% 5.00%  
Discount rate 3.50% 7.30%