Acquisitions, Dispositions, and Impairment (Narrative) (Details) $ in Thousands	1 Months Ended							3 Months Ended			6 Months Ended
	Jun. 30, 2015 USD ($) a	May. 31, 2015 USD ($) a	Mar. 31, 2015 USD ($)	Jan. 31, 2015 USD ($)	Apr. 30, 2014 USD ($) a	Mar. 31, 2014 USD ($) a	Jan. 31, 2014 USD ($) a	Jun. 30, 2015 USD ($) a	Mar. 31, 2015	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) a	Jun. 30, 2014 USD ($)
Area of Land | a	2.7	49.6			7.6		2.9	2.7			2.7
Payments to Acquire Land	$ 9,500	$ 36,300			$ 23,800		$ 15,600
Business Acquisition, Percentage of Voting Interests Acquired			0.01%						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								$ 9,500	$ 0
Impairment associated with land holdings								$ 0		$ 1,152	$ 0	$ 1,152
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Area of Land | a					4.7	3.0
Sales price of land parcels			400		$ 8,300	$ 6,300
Gain (loss) on disposition of land			$ 100		$ 1,400	$ 400
Number of operating properties sold				2
Number of Apartment Units in operating Property Disposed				1,116
Proceeds from sale of operating properties				$ 114,400
Gain (Loss) on Sale of Properties				$ 85,100