Stock-Based Compensation - Option Activity under the Company's Stock Option Plans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Balance at beginning of period	135,250
Granted	0
Exercised	(33,023)
Expired	0
Forfeited	(11)
Balance at end of period	102,216	135,250
Options exercisable at year-end	102,216
Weighted Average Exercise Price
Balance beginning of period	$ 8.79
Granted	0.00
Exercised	9.72
Expired	0.00
Forfeited	7.97
Balance end of period	8.49	$ 8.79
Options exercisable at year-end	$ 8.49
Outstanding, Weighted average remaining contractual term (in years), Ending balance	4 years 3 months 7 days	4 years 2 months 5 days
Options exercisable, Weighted average remaining contractual term (in years), Ending balance	4 years 3 months 7 days
Outstanding, Aggregate intrinsic value	$ 1,101,806	$ 1,450,533
Options exercisable at year end, Aggregate intrinsic value	$ 1,101,806