Acquisitions, Dispositions, Discontinued Operations and Assets Held for Sale (Narrative) (Details) $ in Thousands	1 Months Ended						3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) a	Jun. 30, 2016 USD ($) a	Apr. 30, 2016 USD ($)	Feb. 29, 2016 USD ($) a	Mar. 31, 2015 USD ($)	Jan. 31, 2015 USD ($)	Sep. 30, 2016 USD ($) a	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) a	Sep. 30, 2015 USD ($)	Apr. 26, 2016 a	Jun. 30, 2015 a	May 31, 2015 a
Area of Land | a	2.4	2.2					2.4		2.4			2.7	49.6
Payments to Acquire Land	$ 15,000	$ 4,900								$ 45,800
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax							$ 0	$ 0	$ 375,237	$ 0
Restricted Cash and Cash Equivalents		$ 13,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Area of Land | a				6.3
Number of Units in Real Estate Property	2,906	278				1,116	2,906		2,906
Proceeds from sales of operating properties		$ 39,000				$ 114,400	$ 484,400
Sales price of land				$ 2,200	$ 400
Gain on sale of land				$ 400	$ 100
Number of operating properties sold	5	1				2	5		5
Gain on Sale of Operating Properties		$ 32,200				$ 85,100	$ 262,700
Dual-Phase Property Sale [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Number of operating properties sold	1						1		1
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Area of Land | a											19.6
Number of Units in Real Estate Property											4,918
Proceeds from sales of operating properties			$ 630,000
Number of operating properties sold											15
Weighted average age of operating properties											23