CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) (USD $)	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Number of shares withheld to satisfy employees' tax withholding and other liabilities	129,176	46,852
Shares withheld to satisfy employees' tax withholding and other liabilities, value	$ 14,825,000	$ 3,990,000
Number of shares forfeited	505	1,300
Shares forfeited, value	16,000	39,000
Number of options granted for common stock	144,827	126,484
Value of options granted for common stock	4,258,000	2,460,000
Common dividends declared but not paid	84,815,000	76,127,000
Amount of increase (decrease) in redeemable noncontrolling interests	2,306,000	5,305,000
Repayments of variable-rate, tax-exempt bond financing	93,440,000
Capital Lease Obligations Expenses Incurred	14,500,000
Noncontrolling interest recognized in conjunction with the consolidation of a development joint venture/Fund I subsidiary	3,350,000	4,812,000
Stock Grants
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Number of shares of common stock issued	499,461	102,984
Value of common stock issued	63,147,000	7,777,000
Dividend Reinvestment
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Number of shares of common stock issued	2,548	4,716
Value of common stock issued	310,000	419,000
Deferred Stock Awards
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Number of shares of common stock issued		61,055
Value of common stock issued		3,322,000
Common stock issued, net value		7,489,000
Redeemable Noncontrolling Interests Temporary Equity
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Units of limited partnership	7,500	25
Units of limited partnership, value	365,000	3,000
Cash Flow Hedges
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Corresponding increase (decrease) in other comprehensive income due to a change in other liabilities	(79,691,000)	(448,000)
Fair Value Hedges
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Increase (decrease) in prepaid expenses and other assets due to hedging activities	(1,324,000)	2,181,000
4.75% Fixed Rate Mortgage Loan Due in August 2015
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Mortgage note assumed	44,044,000
5.24 % Fixed Rate Mortgage Loan Due in June 2013
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Mortgage note assumed	55,400,000
5.86% Fixed Rate Mortgage Due in May 2019
SUPPLEMENTAL SCHEDULES OF NONCASH INVESTING AND FINANCING ACTIVITIES
Mortgage note relinquished in non-monetary exchange transaction	$ 55,800,000