CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental disclosures of non-cash investing and financing activities
Number of shares withheld to satisfy employees' tax withholding and other liabilities	121,351	129,762	48,882
Shares withheld to satisfy employees' tax withholding and other liabilities, value	$ 15,543,000	$ 14,891,000	$ 4,203,000
Number of shares and options forfeited	7,558	505	1,300
Shares and options forfeited, value	393,000	35,000	39,000
Number of options granted for common stock	115,303	144,827	126,484
Options granted for common stock, value	3,357,000	4,258,000	2,460,000
Decrease in other comprehensive income due to a change in other liabilities	22,876,000	85,845,000
Cash flow hedge losses reclassified to earnings	1,889,000
Increase (decrease) in other comprehensive income due to a change in prepaid expenses and other assets			(108,000)
Increase (decrease) in prepaid expenses and other assets due to hedge accounting	(11,000)	(1,498,000)	1,737,000
Common dividends declared but not paid	110,966,000	84,953,000	76,676,000
Increase in redeemable noncontrolling interests	375,000	2,607,000	5,573,000
Stock issued in connection with stock grants (in shares)	96,592	511,817	102,984
Stock issued in connection with stock grants	12,883,000	64,747,000	7,777,000
Common stock issued through the dividend reinvestment plan (in shares)	2,331	3,343	5,687
Common stock issued through the dividend reinvestment plan	321,000	403,000	525,000
Redemption/Conversion of DownREIT (in units)		7,500	25
Redemption/Conversion of DownREIT		365,000	3,000
Shares issued to members of Company's Board of Directors (in shares)			61,055
Shares issued, net value		50,224,000	7,385,000
Shares issued to members of Company's Board of Directors, value			3,322,000
Repayments of variable-rate, tax-exempt bond financing		93,440,000
Noncontrolling interest recognized in conjunction with the consolidation of a development joint venture/Fund I subsidiary		3,350,000	4,045,000
4.61% Fixed Rate Mortgage Loan
Supplemental disclosures of non-cash investing and financing activities
Mortgage notes assumed	11,958,000
Stated interest rate (as a percent)	4.61%
4.75% Fixed Rate Mortgage Loan
Supplemental disclosures of non-cash investing and financing activities
Mortgage notes assumed		44,044,000
Stated interest rate (as a percent)		4.75%
5.24% Fixed Rate Mortgage Loan
Supplemental disclosures of non-cash investing and financing activities
Mortgage notes assumed		55,400,000
Stated interest rate (as a percent)		5.24%
5.86% Fixed Rate Mortgage Loan
Supplemental disclosures of non-cash investing and financing activities
Mortgage loans relinquished		55,800,000
Stated interest rate (as a percent)		5.86%
Development Community
Supplemental disclosures of non-cash investing and financing activities
Capital lease obligation recorded in accrued expenses and other liabilities		14,500,000
Development Right
Supplemental disclosures of non-cash investing and financing activities
Capital lease obligation recorded in accrued expenses and other liabilities		$ 17,285,000