Subsequent Events -Additional Information (Detail)			12 Months Ended
	Feb. 24, 2016 USD ($) $ / shares shares	Feb. 18, 2016 USD ($) a ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 a	Dec. 31, 2015 ft²
Subsequent Event [Line Items]
Area of a real estate property					160	1,700,000
Acquisition price of real state property			$ 268,970,875
Costs of shares repurchased			6,484,844	$ 927,912
Repurchase of common stock, authorized amount			$ 10,000,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Area of office building acquired | ft²		4,685
Area of a real estate property | a		0.37
Weighted average remaining lease term		8 years 3 months 18 days
Acquisition price of real state property		$ 2,500,000
Number of shares repurchased | shares	24,024
Costs of shares repurchased	$ 1,100,000
Average price per share | $ / shares	$ 46.21