Subsequent Events - Additional Information (Detail)					1 Months Ended	6 Months Ended	9 Months Ended
	Oct. 27, 2016 USD ($) $ / shares shares	Oct. 14, 2016 USD ($) a ft² tenant	Oct. 13, 2016 USD ($) a $ / shares $ / a	Feb. 18, 2016	Oct. 28, 2016 USD ($) a item	Jun. 30, 2016 shares	Sep. 30, 2016 USD ($) a $ / shares	Sep. 30, 2015 USD ($)	Sep. 30, 2016 ft²	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) a	Dec. 31, 2014 USD ($)
Subsequent Event [Line Items]
Costs of shares repurchased							$ 5,484,295
Area of a real estate property							1,581		1,500,000		148.35
Total acquisition cost of property							$ 49,800,000	$ 34,200,000
Occupancy Percentage				100.00%
Proceeds from land sold							$ 49,253,982	$ 15,226,084
Santa Clara, California
Subsequent Event [Line Items]
Total previous renovation amount												$ 14,000,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Area of a real estate property | a					4,054
Subsequent Event [Member] | Santa Clara, California
Subsequent Event [Line Items]
Area of a real estate property | a		5.24
Area of Building | ft²		76,000
Number of Tenants, Leased Property | tenant		2
Total acquisition cost of property		$ 30,000,000
Occupancy Percentage		100.00%
Additional area density allowable under current zoning | ft²		100,000
New Repurchase Program [Member]
Subsequent Event [Line Items]
Repurchase of common stock, authorized amount										$ 10,000,000	$ 10,000,000
Number of shares repurchased | shares						113,429
Average price per share | $ / shares							$ 48.35
New Repurchase Program [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of shares repurchased | shares	12,196
Costs of shares repurchased	$ 613,000
Average price per share | $ / shares	$ 50.25
2016 Disposals | Sold
Subsequent Event [Line Items]
Sales price										$ 74,300,000
2016 Disposals | Sold | Subsequent Event [Member]
Subsequent Event [Line Items]
Area of a real estate property | a			17
Sales price			$ 3,000,000
Sales price per acre | $ / a			174,000
Gain (loss) on disposal			$ 2,700,000
Gain (loss) on sale of building per share after tax | $ / shares			$ 0.29
2015 Disposals | Sold | Subsequent Event [Member]
Subsequent Event [Line Items]
Proceeds from land sold					$ 1,600,000
Number of buyers | item					2
Centrify Corporation | Subsequent Event [Member] | Santa Clara, California
Subsequent Event [Line Items]
Weighted Average Remaining Lease Term		4 years
Adesto Technologies | Subsequent Event [Member] | Santa Clara, California
Subsequent Event [Line Items]
Weighted Average Remaining Lease Term		7 years