Summary of Critical and Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Principles of consolidation [Abstract]
Noncontrolling interest in Operating Partnership (in hundredths)	6.20%	6.60%
Number of DownREIT limited partnerships the company consolidates	19
Number of communities within the DownREIT partnership	12
Assets related to variable interest entities, net intercompany eliminations	$ 215,200,000	$ 217,300,000
Liabilities related to variable interest entities, net of intercompany eliminations	173,400,000	168,000,000
Redemption rights of limited partners by issuance of common stock for partnership unit, conversion basis	one share per unit basis
Total DownREIT units outstanding (in units)	1,063,848	1,096,871
Redemption value of the variable interest entities	149,500,000	125,300,000
Real estate rental properties [Abstract]
Threshold useful life of assets for capitalization (in years)	1Y
Depreciable life of various categories of fixed assets [Abstract]
Expected lease term of acquired tenant (in months)	20M
Written-off development costs			6,700,000
Number of land parcels no longer developed by Company	2
Revenues and gains on sale of real estate [Abstract]
Lease terms, minimum (in months)	6M
Lease terms, maximum (in months)	12M
No rent time period, minimum (in months)	1M
No rent time period, maximum (in months)	2M
Cash equivalents and restricted cash [Abstract]
Original maturities of highly liquid investments to be classified as cash equivalents, maximum (in months)	3M
Marketable securities [Abstract]
Maturity date of investments in mortgage backed securities, first	November 2019	November 2019
Maturity date of investments in mortgage backed securities, final	September 2020	September 2020
Proceeds from sales of available for sale securities	33,000,000	102,000,000	23,000,000
Gain from sales of available-for-sale securities	5,000,000	12,400,000	1,000,000
Fair value of financial instruments [Abstract]
Fixed rate debt, carrying amount	1,770,000,000	1,560,000,000
Fixed rate debt, fair value	1,880,000,000	1,580,000,000
Variable rate debt, carrying amount	593,700,000	695,200,000
Variable rate debt, fair value	572,300,000	672,800,000
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	72,203,000	87,213,000
Gross Unrealized Gain(Loss)	2,072,000	5,097,000
Fair Value	74,275,000	92,310,000
Total Amortized Cost	46,738,000	41,987,000
Total Gross Unrealized Gain (Loss)	0	0
Total Fair Value	46,738,000	41,987,000
Interest income	10,501,000	15,350,000	11,841,000
Gains on sales of marketable securities	4,956,000	12,491,000	1,014,000
Tax benefit - Taxable REIT Subsidiary	1,682,000	0	0
Other income	0	0	185,000
Total interest and other income	17,139,000	27,841,000	13,040,000
Investment-grade unsecured bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,615,000	22,243,000
Gross Unrealized Gain(Loss)	399,000	4,403,000
Fair Value	4,014,000	26,646,000
Investment funds-US treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,783,000	14,345,000
Gross Unrealized Gain(Loss)	121,000	582,000
Fair Value	11,904,000	14,927,000
Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,067,000	8,638,000
Gross Unrealized Gain(Loss)	1,552,000	112,000
Fair Value	$ 11,619,000	$ 8,750,000
Common Stock [Member]
Status of cash dividends distributed [Abstract]
Ordinary income (in hundredths)	63.68%	82.46%	79.82%
Capital gains (in hundredths)	11.16%	5.61%	15.76%
Unrecaptured section 1250 capital gain (in hundredths)	0.74%	0.00%	4.42%
Return of capital (in hundredths)	24.42%	11.93%	0.00%
Total cash dividends distributed (in hundredths)	100.00%	100.00%	100.00%
Series F and G Preferred Stock [Member]
Status of cash dividends distributed [Abstract]
Ordinary income (in hundredths)	100.00%	93.63%	79.82%
Capital gains (in hundredths)	0.00%	6.37%	15.76%
Unrecaptured section 1250 capital gain (in hundredths)	0.00%	0.00%	4.42%
Return of capital (in hundredths)	0.00%	0.00%	0.00%
Total cash dividends distributed (in hundredths)	100.00%	100.00%	100.00%
Computer Software and Equipment [Member]
Depreciable life of various categories of fixed assets [Abstract]
Depreciable life, minimum (in years)	3
Depreciable life, maximum (in years)	5
Interior Unit Improvements [Member]
Depreciable life of various categories of fixed assets [Abstract]
Depreciable life, average (in years)	5
Land Improvements and Certain Exterior Components of Real Property [Member]
Depreciable life of various categories of fixed assets [Abstract]
Depreciable life, average (in years)	10
Real Estate Structures [Member]
Depreciable life of various categories of fixed assets [Abstract]
Depreciable life, average (in years)	30