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Earnings per Common Share - Additional Information (Detail) - shares
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Earnings Per Share [Abstract]        
Dilutive shares related to convertible preferred stock 1,643,980 2,426,565 2,054,825 2,455,008