Property (Narrative) (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) property	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) property
Real Estate [Abstract]
Number of centers sold | property	8
Proceeds from sale and disposition of property	$ 79,100
Gain on sale of property	$ 43,917	$ 115,989
Number of acquired centers | property	5
Gross acquisition purchase price	$ 208,000
Investment in new development	$ 22,000
Number of properties held for sale | property	1		1
Property held for sale	$ 6,300		$ 9,400