Real Estate Acquired and Sold - Narrative (Details)	1 Months Ended						9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) ft²	Feb. 28, 2015 USD ($) a	Dec. 31, 2014 USD ($) property	Aug. 31, 2014 USD ($)	Apr. 30, 2014 USD ($) ft²	Jan. 31, 2014 USD ($)	Sep. 30, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) property	Mar. 31, 2014 USD ($)
Business Acquisition [Line Items]
Aggregate cost								$ 68,000,000
Property acquisition costs			$ 900,000					$ 900,000
Area of real estate property (in sq ft/acre) | ft²	197,127						197,127
Number of properties purchased | property			5					5
Olney Center
Business Acquisition [Line Items]
Name of property					Olney Center
Location of property					Olney, Maryland
Proceeds from sale of real estate					$ 11,000,000		$ 11,000,000
Area of real estate property (in sq ft/acre) | ft²					53,765
Lease term					20 years
Sale leaseback, option to purchase the property	$ 14,600,000				$ 14,600,000		14,600,000
Sales price of property	11,000,000						$ 11,000,000
Annual increase in rental payments, percentage							1.50%
Giant Center
Business Acquisition [Line Items]
Name of property					Giant Center
Location of property					Milford Mill, Maryland
Proceeds from sale of real estate					$ 7,500,000
Area of real estate property (in sq ft/acre) | ft²					70,040
Gain on sale of real estate					$ 6,100,000
Assets held-for-sale									$ 500,000
Liabilities of assets held-for-sale									$ 100,000
1580 Rockville Pike
Business Acquisition [Line Items]
Aggregate cost						$ 8,000,000
Name of property						1580 Rockville Pike
Property acquisition costs						$ 200,000
1582 Rockville Pike
Business Acquisition [Line Items]
Aggregate cost					$ 11,000,000
Name of property					1582 Rockville Pike
Property acquisition costs					$ 200,000
Location of property					Rockville, Maryland
1584 Rockville Pike
Business Acquisition [Line Items]
Aggregate cost			$ 6,200,000
Name of property			1584 Rockville Pike
Property acquisition costs			$ 200,000					$ 200,000
750 N. Glebe Road
Business Acquisition [Line Items]
Aggregate cost				$ 40,000,000
Name of property				750 N. Glebe Road
Property acquisition costs				$ 400,000
730 N. Glebe Road
Business Acquisition [Line Items]
Aggregate cost			$ 2,800,000
Name of property			730 N. Glebe Road
Property acquisition costs			$ 40,400					$ 40,400
726 N. Glebe Road
Business Acquisition [Line Items]
Aggregate cost	$ 4,000,000
Name of property	726 N. Glebe Road
Property acquisition costs	$ 56,700						$ 56,700
Westview Pad
Business Acquisition [Line Items]
Aggregate cost		$ 900,000
Name of property		Westview Pad
Location of property		Frederick, Maryland
Area of real estate property (in sq ft/acre) | a		1.1