Property Dispositions (Narratives) (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) property	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) property	Sep. 30, 2015 USD ($)	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	$ 82,287	$ 0	$ 132,702	$ 2,197
Payments of mortgage principal			155,183	114,867
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
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	22		34
Proceeds from sales of property	$ 151,300		$ 259,100
Gain on sale of real estate, net of tax	82,300		132,700
Payments of mortgage principal	41,800		84,700
Loss on extinguishment of debt	8,218	0	15,807	0
Impairment charges	$ 29,183	$ 0	$ 29,183	$ 0