Organization and Basis of Presentation (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Organization and Basis of Presentation [Abstract]
General partner ownership interest (in hundredths)			94.10%
Operating Partnership units outstanding (in shares)	2,234,393		2,234,393		2,229,230
Redemption value of Operating Partnership units outstanding	$ 331,200,000		$ 331,200,000		$ 313,200,000
Number of apartment communities owned	164		164
Apartment units owned (in units)	33,637		33,637
Ownership interests, number of commercial buildings	5		5
Ownership interests, number of active development projects	9		9
Available-for-sale Securities, Gross Realized Gain (Loss), Disclosures [Abstract]
Available-for-sale securities, sales			5,100,000	28,000,000
Gains on sales of marketable securities			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)	1,057,848		1,057,848		1,063,848
Redemption value of the variable interest entities	156,800,000		156,800,000		149,500,000
Assets related to variable interest entities, net intercompany eliminations	202,700,000		202,700,000		176,700,000
Liabilities related to variable interest entities, net of intercompany eliminations	199,800,000		199,800,000		171,500,000
Stock-Based Compensation [Abstract]
Stock-based compensation expense for options and restricted stock	400,000	300,000	1,200,000	1,000,000
Intrinsic value of the stock options exercised	500,000	600,000	2,400,000	3,600,000
Intrinsic value of the stock options outstanding and fully vested	13,600,000		13,600,000
Unrecognized compensation cost related to unvested share-based compensation granted under the stock option and restricted stock plans	4,300,000		4,300,000
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	200,000	1,600,000	700,000
Capitalization of stock based compensation for Z units	100,000	100,000	400,000	300,000
Intrinsic value of Series Z Units outstanding	22,000,000		22,000,000
Total unrecognized compensation cost related to Z-1 Units subject to future vesting	8,300,000		8,300,000
Unamortized cost recognition period range (in years)			14 years
Fair Value of Financial Instruments [Abstract]
Fixed rate debt and unsecured bonds	2,130,000,000		2,130,000,000
Fair value of fixed rate debt and unsecured bonds	2,260,000,000		2,260,000,000
Variable rate debt instruments excluding borrowings under the lines of credit	402,100,000		402,100,000
Fair value of variable rate debt excluding borrowing under lines of credit	380,900,000		380,900,000
Capitalization Policy [Abstract]
Real Estate Capitalized Internal Development Costs	1,500,000	1,300,000	4,500,000	4,500,000
Real Estate Capitalized Internal Salary Costs	500,000	600,000	1,800,000	1,700,000
Cost / Amortized Cost [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Amortized Cost	143,946,000		143,946,000		72,203,000
Cost / Amortized Cost [Member] | Investment-Grade Unsecured Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	5,191,000		5,191,000		3,615,000
Cost / Amortized Cost [Member] | Investment Funds - US Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	15,379,000		15,379,000		11,783,000
Cost / Amortized Cost [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	72,752,000		72,752,000		10,067,000
Cost / Amortized Cost [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	50,624,000		50,624,000		46,738,000
Gross Unrealized Gain (Loss) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Gross Unrealized Gain (Loss)	(2,373,000)		(2,373,000)		2,072,000
Gross Unrealized Gain (Loss) [Member] | Investment-Grade Unsecured Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gain (Loss)	79,000		79,000		399,000
Gross Unrealized Gain (Loss) [Member] | Investment Funds - US Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gain (Loss)	755,000		755,000		121,000
Gross Unrealized Gain (Loss) [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gain (Loss)	(3,207,000)		(3,207,000)		1,552,000
Gross Unrealized Gain (Loss) [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Gain (Loss)	0		0		0
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fair Value	141,573,000		141,573,000		74,275,000
Carrying Value [Member] | Investment-Grade Unsecured Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	5,270,000		5,270,000		4,014,000
Carrying Value [Member] | Investment Funds - US Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	16,134,000		16,134,000		11,904,000
Carrying Value [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	69,545,000		69,545,000		11,619,000
Carrying Value [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 50,624,000		$ 50,624,000		$ 46,738,000