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Income Per Common Share - Additional Information (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Options      
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Antidilutive Securities Excluded from Computation of Diluted Income Per Common Share 1,190,261 1,335,113 1,467,857