Organization and Basis of Presentation (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013 Apartment Partnership Community Building Project	Jun. 30, 2012	Jun. 30, 2013 Apartment Partnership Community Building Project	Jun. 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)	21,479,701		21,479,701		2,122,381
Redemption value of Operating Partnership units outstanding	$ 341,400,000		$ 341,400,000		$ 311,200,000
Number of apartment communities owned	167		167
Apartment units owned (in units)	34,514		34,514
Ownership interests, number of commercial buildings	5		5
Ownership interests, number of active development projects	9		9
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	88,392,000		88,392,000		90,182,000
Total Gross Unrealized Gain	342,000		342,000		2,531,000
Total Carrying Value	88,734,000		88,734,000		92,713,000
Proceeds from sales of available for sale securities	0	5,100,000	20,300,000	5,100,000
Gain from sales of available-for-sale securities	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)	1,029,095		1,029,095		1,039,431
Redemption value of the variable interest entities	163,500,000		163,500,000		152,400,000
Assets related to variable interest entities, net intercompany eliminations	203,000,000		203,000,000		201,100,000
Liabilities related to variable interest entities, net of intercompany eliminations	183,400,000		183,400,000		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	1,100,000	800,000
Intrinsic value of the stock options exercised	1,900,000	700,000	2,800,000	1,800,000
Intrinsic value of the stock options outstanding and fully vested	17,700,000		17,700,000
Unrecognized compensation cost related to unvested share-based compensation granted under the stock option and restricted stock plans	5,200,000		5,200,000
Stock-based compensation expense for the Series Z Units	500,000	500,000	1,000,000	1,100,000
Capitalization of stock based compensation for Z units	100,000	200,000	200,000	300,000
Intrinsic value of Series Z Units outstanding	17,000,000		17,000,000
Total unrecognized compensation cost related to Z-1 Units subject to future vesting	6,300,000		6,300,000
Unamortized cost recognition period range (in years)	14 years
Fair Value of Financial Instruments [Abstract]
Fixed rate debt carrying amount	2,380,000,000		2,380,000,000
Fixed rate debt fair value	2,430,000,000		2,430,000,000
Variable rate debt, carrying amount	537,300,000		537,300,000
Variable rate debt fair value	517,600,000		517,600,000
Capitalization Policy [Abstract]
Capitalized internal costs related to development and redevelopment projects	1,700,000	1,500,000	3,300,000	2,900,000
Capitalized salaries	600,000	600,000	1,200,000	1,300,000
Change in fair value and amortization of derivatives [Abstract]
Balance at beginning			(71,658,000)
Other comprehensive income (loss) before reclassification, derivative	4,528,000
Amounts reclassified from accumulated other comprehensive loss	4,023,000
Net other comprehensive income (loss)	8,551,000
Balance at the end	(63,107,000)		(63,107,000)
Unrealized gains/(losses) on available for sale securities [Abstract]
Balance at beginning	294,000		294,000		2,397,000
Other comprehensive income (loss) before reclassification, available for sale securities, total	(430,000)
Amounts reclassified from accumulated other comprehensive loss	(1,673,000)
Net other comprehensive income (loss)	(2,103,000)
Balance at the end	294,000		294,000		2,397,000
Accumulated other comprehensive loss, net by component [Abstract]
Balance at beginning	(62,813,000)		(62,813,000)		(69,261,000)
Other comprehensive income (loss) before reclassification	4,098,000
Amounts reclassified from accumulated other comprehensive loss	2,350,000
Net other comprehensive income (loss)	6,448,000
Balance at the end	(62,813,000)		(62,813,000)		(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	5,083,000		5,083,000		5,143,000
Gross Unrealized Gain	62,000		62,000		98,000
Carrying Value	5,145,000		5,145,000		5,241,000
Investment Funds - US Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	15,360,000		15,360,000		14,120,000
Gross Unrealized Gain	549,000		549,000		729,000
Carrying Value	15,909,000		15,909,000		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	(269,000)		(269,000)		1,704,000
Carrying Value	12,835,000		12,835,000		20,621,000
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	54,845,000		54,845,000		52,002,000
Gross Unrealized Gain	0		0		0
Carrying Value	$ 54,845,000		$ 54,845,000		$ 52,002,000