Treasury Stock - Additional Information (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Nov. 30, 2008
Equity, Class of Treasury Stock [Line Items]
Number of shares purchased	222,624	159,873
Costs of shares purchased	$ 10,864,945	$ 7,866,410
Odd-Lot Buy-Back Program [Member]
Equity, Class of Treasury Stock [Line Items]
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 purchased		119,403	25,836
Costs of shares purchased		$ 6,500,000	$ 928,000
Average price per share		$ 54.31	$ 35.92
New Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock, authorized amount	10,000,000	$ 10,000,000
Costs of shares purchased	$ 3,000,000
Average price per share	$ 47.78
Number of shares repurchased	62,751