Share-Based Payments - Activity of Exercisable Common Stock Options (Detail) - Common Stock Options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Exercisable common stock options, beginning balance	748,150	775,650	986,300
Common stock options exercised	(750)	(27,500)	(242,050)
Common stock options vested			31,400
Common stock options expired	(379,800)
Exercisable common stock options, ending balance	367,600	748,150	775,650
Weighted Average Exercise Price Per Share
Exercisable common stock options, beginning balance	$ 6.33	$ 6.28	$ 6.08
Common stock options exercised	3.00	4.76	5.16
Common stock options vested			3.84
Common stock options expired	5.65
Exercisable common stock options, ending balance	$ 7.06	$ 6.33	$ 6.28