Treasury Stock - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 26, 2012	Nov. 30, 2008
Equity, Class of Treasury Stock [Line Items]
Number of shares purchased	159,873	159,873	40,470
Costs of shares purchased	$ 7,866,410	$ 7,866,410	$ 1,381,566
Repurchase of common stock, authorized amount	$ 10,000,000	10,000,000
Costs of shares repurchased		$ 6,484,844	$ 927,912
Odd-Lot Buy-Back Program [Member]
Equity, Class of Treasury Stock [Line Items]
Description of number of shares held by shareholder, maximum		less than 100
Purchase price of shares from shareholders					$ 31.00
Number of shares purchased	119,403	119,403	25,836		14,634
Costs of shares purchased	$ 6,500,000	$ 6,500,000	$ 928,000		$ 454,000
Buy back expiration date		Jun. 30, 2012
Repurchase of common stock, authorized amount						$ 8,000,000
Number of shares retired			4,660
Amount of treasury shares retired				$ 105,000
Number of shares repurchased	164,533
Average price per share	$ 48.45	$ 54.31	$ 35.92
Completion Of Eight Million Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	49,167
Costs of shares repurchased	$ 2,600,000
Average price per share	$ 53.44
New Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock, authorized amount	$ 10,000,000	$ 10,000,000
Number of shares repurchased		0
Maximum [Member] | Odd-Lot Buy-Back Program [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares held by shareholders					100