Real estate assets held for sale (Details)	6 Months Ended
	Jun. 30, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Document Period End Date	Jun. 30, 2015
Operating properties accumulated depreciation, held for sale	$ 13,294,000
Operating properties, net held for sale	74,667,000.000
Notes and accounts receivable held for sale	560,000.000
Deferred rents receivable held for sale	6,195,000.000
Intangible assets, net held for sale	4,391,000.000
Held for sale, other assets	962,000.000
Disposal Group, Including Discontinued Operation, Long Lived Assets	86,775,000	$ 0
Accounts payable and accrued expenses held for sale	2,319,000
Intangible liabilities, net held for sale	128,000
Other liabilities held for sale	685,000
Disposal Group, Including Discontinued Operation, Liabilities, Current	3,132,000	$ 0
Intangible assets accumulated amortization, held for sale	7,590,000
Intangible liabilities accumulated amortization, held for sale	$ 308,000
2100 Ross Avenue [Member]
Long Lived Assets Held-for-sale [Line Items]
Net Rentable Area | ft²	844,000