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Investments - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Bottom of range [member]    
Disclosure of detailed information about investment property [line items]    
Interest rates on time deposits 1.37% 1.00%
Bottom of range [member] | United States of America, Dollars    
Disclosure of detailed information about investment property [line items]    
Interest rate on long term time deposit 3.20%  
Top of range [member]    
Disclosure of detailed information about investment property [line items]    
Interest rates on time deposits 3.75% 3.75%
Top of range [member] | United States of America, Dollars    
Disclosure of detailed information about investment property [line items]    
Interest rate on long term time deposit 3.75%