Real estate assets held for sale (Details)	9 Months Ended
	Sep. 30, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Document Period End Date	Sep. 30, 2015
Operating properties accumulated depreciation, held for sale
Operating properties, net held for sale	$ 44,560,000.000
Notes and accounts receivable held for sale	606,000.000
Deferred rents receivable held for sale	4,239,000.000
Held for sale, other assets	1,086,000.000
Disposal Group, Including Discontinued Operation, Long Lived Assets	50,491,000	$ 0
Accounts payable and accrued expenses held for sale	2,212,000
Other liabilities held for sale	631,000
Disposal Group, Including Discontinued Operation, Liabilities, Current	$ 2,843,000	$ 0
Intangible assets accumulated amortization, held for sale
Intangible liabilities accumulated amortization, held for sale
North Point Center [Member]
Long Lived Assets Held-for-sale [Line Items]
Net Rentable Area | ft²	540,000,000
The Points at Waterview [Member]
Long Lived Assets Held-for-sale [Line Items]
Net Rentable Area | ft²	203,000,000