Organization and Basis of Presentation (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Organization and Basis of Presentation [Abstract]
General partner ownership interest (in hundredths)			94.00%
Operating Partnership units outstanding (in shares)	2,238,571		2,238,571		2,229,230
Redemption value of Operating Partnership units outstanding	$ 344,600,000		$ 344,600,000		$ 313,200,000
Number of apartment communities owned	160		160
Apartment units owned (in units)	33,015		33,015
Ownership interests, number of commercial buildings	5		5
Ownership interests, number of active development projects	9		9
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Amortized Cost	112,874,000		112,874,000		72,203,000
Total Gross Unrealized Gain (Loss)	1,292,000		1,292,000		2,072,000
Total Fair Value	114,166,000		114,166,000		74,275,000
Available-for-sale Securities, Gross Realized Gain (Loss), Disclosures [Abstract]
Available-for-sale securities, sales	5,100,000		5,100,000	26,800,000
Gains on sales of marketable securities	500,000		500,000	4,500,000
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)	1,061,848		1,061,848		1,063,848
Redemption value of the variable interest entities	163,400,000		163,400,000		149,500,000
Assets related to variable interest entities, net intercompany eliminations	201,400,000		201,400,000		199,800,000
Liabilities related to variable interest entities, net of intercompany eliminations	174,000,000		174,000,000		171,500,000
Stock-Based Compensation [Abstract]
Stock-based compensation expense for options and restricted stock	400,000	400,000	800,000	700,000
Intrinsic value of the stock options exercised	700,000	2,000,000	1,800,000	3,000,000
Intrinsic value of the stock options outstanding and fully vested	16,300,000		16,300,000
Unrecognized compensation cost related to unvested share-based compensation granted under the stock option and restricted stock plans	4,900,000		4,900,000
Compensation cost recognized for stock options over a weighted-average period			1 to 6 years
Compensation cost for restricted stock awards expected to be recognized straight-line	7		7
Stock-based compensation expense for the Series Z Units	500,000	300,000	1,100,000	500,000
Capitalization of stock based compensation for Z units	200,000	100,000	300,000	200,000
Intrinsic value of Series Z Units outstanding	22,800,000		22,800,000
Unamortized cost recognition period range (in years)	14		14
Total unrecognized compensation cost related to Z-1 Units subject to future vesting	9,200,000		9,200,000
Fair Value of Financial Instruments [Abstract]
Fixed rate debt and unsecured bonds	1,790,000,000		1,790,000,000
Fair value of fixed rate debt and unsecured bonds	1,900,000,000		1,900,000,000
Variable rate debt instruments excluding borrowings under the lines of credit	439,000,000		439,000,000
Fair value of variable rate debt excluding borrowing under lines of credit	417,700,000		417,700,000
Capitalization Policy [Abstract]
Real Estate Capitalized Internal Development Costs	1,100,000	1,100,000	2,200,000	2,000,000
Real Estate Capitalized Internal Salary Costs	600,000	600,000	1,300,000	1,100,000
Investment-Grade Unsecured Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	5,227,000		5,227,000		3,615,000
Gross Unrealized Gain (Loss)	(22,000)		(22,000)		399,000
Fair Value	5,205,000		5,205,000		4,014,000
Investment Funds - US Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	15,383,000		15,383,000		11,783,000
Gross Unrealized Gain (Loss)	593,000		593,000		121,000
Fair Value	15,976,000		15,976,000		11,904,000
Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	42,970,000		42,970,000		10,067,000
Gross Unrealized Gain (Loss)	721,000		721,000		1,552,000
Fair Value	43,691,000		43,691,000		11,619,000
Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	49,294,000		49,294,000		46,738,000
Gross Unrealized Gain (Loss)	0		0		0
Fair Value	$ 49,294,000		$ 49,294,000		$ 46,738,000