Property Dispositions (Narratives) (Details) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Aug. 01, 2016 USD ($) property	Jun. 30, 2016 USD ($) property	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) property	Jun. 30, 2015 USD ($)	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations
Gain on sale of real estate, net of tax		$ 25,017	$ 0	$ 50,415	$ 2,197
Payments of mortgage principal				$ 57,969	36,139
Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations
Number of properties sold | property		9		12
Proceeds from sales of property		$ 61,300		$ 107,700
Gain on sale of real estate, net of tax		25,000		50,400
Loss on extinguishment of debt		5,091	$ 0	7,590	$ 0
Payments of mortgage principal		27,900		42,900
Ishops
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations
Percentage of gain deferred						15.00%
Deferred gain on the sale of property		$ 2,200		$ 2,200
Self storage
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations
Number of properties held for sale, count | property		22		22
Subsequent Event | Self storage
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations
Number of properties sold | property	22
Proceeds from sales of property	$ 154,400