Acquisitions, Dispositions, Assets Held for Sale and Impairment (Narrative) (Details) $ in Thousands	1 Months Ended											3 Months Ended		9 Months Ended
	Oct. 30, 2015 USD ($) a	Jun. 30, 2015 USD ($) a	May. 31, 2015 USD ($) a	Mar. 31, 2015 USD ($)	Jan. 31, 2015 USD ($)	Aug. 31, 2014 USD ($) a	Jul. 31, 2014 USD ($) a	Jun. 30, 2014 USD ($)	Apr. 30, 2014 USD ($) a	Feb. 28, 2014 USD ($)	Jan. 31, 2014 USD ($) a	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($) a	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($) a
Area of Land | a		2.7	49.6						7.6		2.9
Payments to Acquire Land		$ 9,500	$ 36,300						$ 23,800		$ 15,600
Proceeds from sales of operating properties										$ 65,600
Purchase of non-controlling interests														$ 9,500	$ 0
Impairment associated with land holdings								$ 1,200				$ 0	$ 0	$ 0	$ 1,152
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Area of Land | a						19.2	2.4						7.7		7.7
Number of apartment units in wholly owned property acquired					1,116
Proceeds from sales of operating properties					$ 114,400
Sales price of land parcels				$ 400		$ 8,300	$ 800								$ 14,600
Gains (losses) on sales of land parcels				$ 100		$ 1,800									$ 1,800
Number of operating properties sold					2
Gain (Loss) on Sales of Operating Properties					$ 85,100
Subsequent Event [Member]
Area of Land | a	5.8
Payments to Acquire Land	$ 13,300
Subsequent Event [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Number of apartment units in wholly owned property acquired	260
Proceeds from sales of operating properties	$ 33,000
Corporate Joint Venture [Member]
Business Acquisition, Percentage of Voting Interests Acquired				0.01%
Number of fully consolidated joint ventures acquired				2
Number of apartment units in wholly owned property acquired				798
Purchase of non-controlling interests				$ 9,500