Assets classified as "held for sale" (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft² property	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Net assets of discontinued operations
Investments in real estate	$ 138,200		$ 138,200		$ 173,706
Other assets	6,659		6,659		10,147
Total assets	144,859		144,859		183,853
Total liabilities	0		0		(6,044)
Net assets classified as held for sale	144,859		144,859		$ 177,809
Assets held for sale [Line Items]
Total revenues	3,454	$ 3,108	9,694	$ 9,418
Disposal Group, Including Assets Held for Sale Not Qualifying as Discontinued Operations, Operating Expense	(1,163)	(1,134)	(3,293)	(2,971)
Total revenues less operating expenses from assets held for sale	2,291	1,974	6,401	6,447
Depreciation expense	0	(2,025)	(503)	(6,103)
Impairment of real estate	0	(295)	(14,510)	(295)
Income (loss) from assets held for sale	$ 2,291	$ (346)	$ (8,612)	$ 49
Disposal Group Including Assets Held for Sale, Not Qualifying as Discontinued Operations, Number of Properties | property			3
Disposal Group Including Assets Held for Sale, Not Qualifying as Discontinued Operations, Area of Real Estate | ft²	317,060		317,060
Assets held for sale sold in 2015 [Member]
Assets held for sale [Line Items]
Disposal Group Including Assets Held for Sale, Not Qualifying as Discontinued Operations, Area of Real Estate | ft²	196,859		196,859
Disposal Group Including Assets Held for Sale, Not Qualifying as Discontinued Operations, Properties Sold Number | property			3