Organization and Basis of Presentation (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Apartment Partnership Community Building Project	Sep. 30, 2012	Sep. 30, 2013 Apartment Partnership Community Building Project	Sep. 30, 2012	Dec. 31, 2012
Organization and Basis of Presentation [Abstract]
General partner ownership interest (in hundredths)	94.60%
Operating Partnership units outstanding (in shares)	2,146,293		2,146,293		2,122,381
Redemption value of Operating Partnership units outstanding	$ 317,000,000		$ 317,000,000		$ 311,200,000
Number of apartment communities owned	163		163
Apartment units owned (in units)	33,542		33,542
Ownership interests, number of commercial buildings	5		5
Ownership interests, number of active development projects	11		11
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	90,224,000		90,224,000		90,182,000
Total Gross Unrealized Gain	(325,000)		(325,000)		2,531,000
Total Carrying Value	89,899,000		89,899,000		92,713,000
Proceeds from sales of available for sale securities	0	0	20,300,000	6,300,000
Gain from sales of available-for-sale securities	0	0	1,800,000	500,000
Variable Interest Entities [Abstract]
Number of DownREIT limited partnerships the company consolidates	19		19
Number of communities within the DownREIT partnership	12		12
Total DownREIT Partnership's Outstanding units (shares)	1,011,071		1,011,071		1,039,431
Redemption value of the variable interest entities	149,300,000		149,300,000		152,400,000
Assets related to variable interest entities, net intercompany eliminations	201,600,000		201,600,000		201,100,000
Liabilities related to variable interest entities, net of intercompany eliminations	185,500,000		185,500,000		178,600,000
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense for options and restricted stock	500,000	400,000	1,600,000	1,200,000
Intrinsic value of the stock options exercised	100,000	500,000	2,900,000	2,400,000
Intrinsic value of the stock options outstanding and fully vested	11,200,000		11,200,000
Unrecognized compensation cost related to unvested share-based compensation granted under the stock option and restricted stock plans	5,000,000		5,000,000
Stock-based compensation expense for the Series Z Units	500,000	500,000	1,500,000	1,600,000
Capitalization of stock based compensation for Z units	100,000	100,000	300,000	400,000
Intrinsic value of Series Z Units outstanding	15,800,000		15,800,000
Total unrecognized compensation cost related to Z-1 Units subject to future vesting	5,600,000		5,600,000
Unamortized cost recognition period range (in years)	14 years
Fair Value of Financial Instruments [Abstract]
Fixed rate debt carrying amount	2,370,000,000		2,370,000,000
Fixed rate debt fair value	2,420,000,000		2,420,000,000
Variable rate debt, carrying amount	552,600,000		552,600,000
Variable rate debt fair value	532,900,000		532,900,000
Capitalization Policy [Abstract]
Capitalized internal costs related to development and redevelopment projects	1,800,000	1,500,000	5,100,000	4,500,000
Capitalized salaries	800,000	500,000	2,000,000	1,900,000
Change in fair value and amortization of derivatives [Abstract]
Balance at beginning			(71,658,000)
Other comprehensive income (loss) before reclassification, derivative			3,075,000
Amounts reclassified from accumulated other comprehensive loss			6,364,000
Net other comprehensive income (loss)			9,439,000
Balance at the end	(62,219,000)		(62,219,000)
Unrealized gains/(losses) on available for sale securities [Abstract]
Balance at beginning	(370,000)		(370,000)		2,397,000
Other comprehensive income (loss) before reclassification, available for sale securities, total			(1,094,000)
Amounts reclassified from accumulated other comprehensive loss			(1,673,000)
Net other comprehensive income (loss)			(2,767,000)
Balance at the end	(370,000)		(370,000)		2,397,000
Accumulated other comprehensive loss, net by component [Abstract]
Balance at beginning	(62,589,000)		(62,589,000)		(69,261,000)
Other comprehensive income (loss) before reclassification			1,981,000
Amounts reclassified from accumulated other comprehensive loss			4,691,000
Net other comprehensive income (loss)			6,672,000
Balance at the end	(62,589,000)		(62,589,000)		(69,261,000)
Minimum [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Range of weighted average period on recognition of compensation cost for stock options	1 year
Range of weighted average period on recognition of compensation cost for restricted stock	1 year
Maximum [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Range of weighted average period on recognition of compensation cost for stock options	5 years
Range of weighted average period on recognition of compensation cost for restricted stock	7 years
Investment-Grade Unsecured Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	5,034,000		5,034,000		5,143,000
Gross Unrealized Gain	80,000		80,000		98,000
Carrying Value	5,114,000		5,114,000		5,241,000
Investment Funds - US Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	15,364,000		15,364,000		14,120,000
Gross Unrealized Gain	570,000		570,000		729,000
Carrying Value	15,934,000		15,934,000		14,849,000
Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	13,104,000		13,104,000		18,917,000
Gross Unrealized Gain	(975,000)		(975,000)		1,704,000
Carrying Value	12,129,000		12,129,000		20,621,000
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	56,722,000		56,722,000		52,002,000
Gross Unrealized Gain	0		0		0
Carrying Value	$ 56,722,000		$ 56,722,000		$ 52,002,000