Share-Based Compensation - Option Activity under the Company's Stock Option Plans (Detail) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Number of Shares
Outstanding, Number of shares, Beginning balance (in shares)	102,216
Granted, Number of shares (in shares)	0	0
Exercised, Number of shares (in shares)	(34,728)
Forfeited, Number of shares (in shares)	0
Expired, Number of shares (in shares)	0
Outstanding, Number of shares, Ending balance (in shares)	67,488		102,216
Options exercisable, Number of shares, Ending balance (in shares)	67,488
Weighted Average Exercise Price
Outstanding, Weighted average exercise price, Beginning balance (in usd per share)	$ 8.49
Granted, Weighted average exercise price (in usd per share)	0.00
Exercised, Weighted average exercise price (in usd per share)	8.84
Forfeited, Weighted average exercise price (in usd per share)	0.00
Expired, Weighted average exercise price (in usd per share)	0.00
Outstanding, Weighted average exercise price, Ending balance (in usd per share)	8.31		$ 8.49
Options exercisable, Weighted average exercise price, Ending balance (in usd per share)	$ 8.31
Weighted Average Remaining Contractual Term (in years)
Outstanding, Average remaining contractual term (in years)	3 years 1 month 10 days		4 years 3 months 7 days
Options exercisable, Average remaining contractual term (in years)	3 years 1 month 10 days
Aggregate Intrinsic Value
Outstanding, Aggregate intrinsic value	$ 659,147		$ 1,101,806
Options exercisable at year end, Aggregate intrinsic value	$ 659,147