Notes Payable and Unsecured Credit Facilities Notes Payable and Unsecured Credit Facilities(Details) (USD $)									12 Months Ended		3 Months Ended	12 Months Ended		12 Months Ended							12 Months Ended		3 Months Ended		12 Months Ended					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Oct. 07, 2010	Jun. 02, 2010	Dec. 31, 2011 Notes Payable to Banks [Member]	Dec. 31, 2010 Notes Payable to Banks [Member]	Dec. 31, 2011 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2010 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2011 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2010 Fixed Rate Mortgage Loans [Member]	Mar. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2010 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member] General Loan [Member]	Dec. 31, 2011 Unsecured Debt [Member] Construction Loans [Member]	Dec. 31, 2011 Unsecured Debt [Member] Term Loan - Unsecured [Member]	Dec. 31, 2011 7.95% Notes Due 1/15/11 [Member]	Dec. 31, 2011 7.25% Notes Due 12/12/11 [Member]	Dec. 31, 2011 Secured Debt [Member]	Dec. 31, 2010 Line of Credit [Member]	Dec. 31, 2011 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Dec. 31, 2010 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Mar. 31, 2011 Line of Credit [Member] Line of Credit Facility - Revolving [Member]	Feb. 28, 2011 Line of Credit [Member] Line of Credit Facility - Revolving [Member]	Dec. 31, 2011 Variable Income Interest Rate [Member] Construction Loans [Member]	Dec. 31, 2011 Scheduled Principal Payments [Member]	Dec. 31, 2011 Mortgage Loan Maturities [Member]	Dec. 31, 2011 Unsecured Maturities [Member]		Dec. 31, 2011 Partnership Interest [Member]	Dec. 31, 2010 Partnership Interest [Member]	Dec. 31, 2009 Partnership Interest [Member]	Dec. 31, 2011 Partnership Interest [Member] Secured Debt [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2009 Parent Company [Member]
Debt Instrument [Line Items]
Secured Debt, Percent Guaranteed by Parent Company																																	.1275
Debt Instrument, Maturity Date Range, End												2021							2028
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum									5.22%			4.80%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum									8.40%			6.75%
Debt, Weighted Average Interest Rate									6.43%			5.59%
Mortgage Loans on Real Estate, Carrying Amount of Mortgages														$ 3,700,000	$ 9,000,000
Debt Instrument, Interest Rate Terms														variable interest rate equal to LIBOR plus 380 basis points		variable interest rate of LIBOR plus 145 basis points	0.0795	.0725							variable interest rate of LIBOR plus 160 basis points
Debt Instrument, Maturity Date											Jan. 31, 2011			Oct. 01, 2014	Sep. 01, 2014	Dec. 15, 2016	Jan. 18, 2011	Dec. 12, 2011
Repayments of Unsecured Debt												161,700,000					161,700,000	20,000,000												181,691,000	209,879,000	116,053,000		181,691,000	209,879,000	116,053,000
Line of Credit Facility, Unsecured, Annual Facility Fee																					annual facility fee of 25 basis points
Line of Credit Facility, Unsecured, Interest Rate Description																					variable interest rate of LIBOR plus 125 basis points
Line of Credit Facility, Revolving, Maximum Borrowing Capacity																					600,000,000			113,800,000
Line of Credit Facility, Revolving, Expiration Date																					September 2015, and includes one, one year extension option		February 2011
Long-term Debt, Amount Outstanding	1,982,440,000	2,094,469,000			1,942,440,000	2,084,469,000	1,489,895,000	1,671,129,000	439,880,000	402,151,000		12,665,000	11,189,000			0										54,991,000	397,554,000	1,529,895,000	[1]
Line of Credit Facility, Maximum Increase Allowed to Borrowing Capacity																					1,000,000,000
Unsecured Line of Credit, Amount Outstanding																				0	40,000,000	10,000,000		0						40,000,000	10,000,000			40,000,000	10,000,000
Maturities of Long-term Debt [Abstract]
2012	199,375,000																									6,998,000	0	192,377,000	[1]
2013	23,326,000																									6,996,000	16,330,000	0	[1]
2014	185,000,000																									6,481,000	28,519,000	150,000,000	[1]
2015	441,482,000																									5,169,000	46,313,000	390,000,000	[1]
2016	19,018,000																									4,857,000	14,161,000	0	[1]
Beyond 5 Years	1,112,536,000																									24,490,000	288,046,000	800,000,000	[1]
Unamortized debt discounts (premiums)	1,703,000																									0	4,185,000	(2,482,000)	[1]
Long-term Debt, Amount Outstanding	1,982,440,000	2,094,469,000			1,942,440,000	2,084,469,000	1,489,895,000	1,671,129,000	439,880,000	402,151,000		12,665,000	11,189,000			0										54,991,000	397,554,000	1,529,895,000	[1]
Debt Instrument, Face Amount			250,000,000	150,000,000												250,000,000
Debt Instrument, maximum increase allowed to borrowing capacity																$ 150,000,000

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