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Employee Benefits - Summary of Trust Assets (Detail)
Dec. 31, 2017
Dec. 31, 2016
Variable return:    
Trust assets fixed and variable return percentage 100.00% 100.00%
Traded Securities [member]    
Fixed return:    
Trust assets fixed return percentage 18.00% 15.00%
Bank instruments [member]    
Fixed return:    
Trust assets fixed return percentage 5.00% 4.00%
Federal government instruments of the respective countries [member]    
Fixed return:    
Trust assets fixed return percentage 62.00% 63.00%
Publicly traded shares [member]    
Variable return:    
Trust assets variable return percentage 15.00% 18.00%