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Employee Benefit Plans (Retiree Medical Coverage) - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]    
Maximum number of years medical, dental and prescription drug coverage to spouse of deceased officer 30 years  
Total liability recorded to provide medical, dental and prescription drug coverage $ 0.1 $ 0.2