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Employee Benefits - Summary of Allocation of Fair Value of Plan Assets of Defined Contribution Plans (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Fair Value Of Plan Assets [Line Items]    
Investments in listed and unlisted equity securities 95.00% 95.00%
Smart and Subsidiaries [Member]    
Disclosure Of Fair Value Of Plan Assets [Line Items]    
Investments in debt and fixed income securities and others 77.00% 70.00%
Investments in listed and unlisted equity securities 23.00% 30.00%
Percentage of investments in pension plan assets 100.00% 100.00%