Notes Payable and Unsecured Line of Credit Notes Payable and Unsecured Line of Credit (Details) (USD $)			6 Months Ended			6 Months Ended												6 Months Ended		3 Months Ended	6 Months Ended		6 Months Ended				3 Months Ended	6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Partnership Interest [Member]	Jun. 30, 2010 Partnership Interest [Member]	Dec. 31, 2010 Partnership Interest [Member]	Jun. 30, 2011 Partnership Interest [Member] Secured Debt [Member]	Jun. 30, 2011 Parent Company [Member]	Jun. 30, 2010 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Jun. 30, 2011 Scheduled Principal Payments [Member]	Jun. 30, 2011 Mortgage Loan Maturities [Member]	Jun. 30, 2011 Unsecured Maturities [Member]		Jun. 30, 2011 Notes Payable to Banks [Member]	Dec. 31, 2010 Notes Payable to Banks [Member]	Jun. 30, 2011 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2010 Fixed Rate Unsecured Loans [Member]	Jun. 30, 2011 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2010 Fixed Rate Mortgage Loans [Member]	Mar. 31, 2011 Unsecured Debt [Member]	Jun. 30, 2011 Unsecured Debt [Member]	Dec. 31, 2010 Unsecured Debt [Member]	Jun. 30, 2011 Unsecured Debt [Member] General Loan [Member]	Jun. 30, 2011 Unsecured Debt [Member] Construction Loans [Member]	Jun. 30, 2011 Secured Debt [Member]	Dec. 31, 2010 Line of Credit [Member]	Jun. 30, 2011 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Jun. 30, 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]	Jun. 30, 2011 Variable Income Interest Rate [Member] Construction Loans [Member]
Debt Instrument [Line Items]
Secured Debt, Percent Guaranteed by Parent Company						.0817
Debt Instrument, Maturity Date Range, End																					2021				2022
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum																		5.22%			4.80%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum																		8.40%			7.95%
Debt, Weighted Average Interest Rate																		6.55%			5.61%
Mortgage Loans on Real Estate, Carrying Amount of Mortgages																							$ 3,900,000	$ 8,900,000
Debt Instrument, Interest Rate Terms																					7.95% ten-year senior		variable interest rate equal to LIBOR plus 380 basis points									variable interest rate of LIBOR plus 300 basis points
Debt Instrument, Maturity Date																				Jan. 31, 2011	Jan. 18, 2011		Oct. 01, 2014	Sep. 02, 2011
Repayments of Unsecured Debt			161,691,000	0			161,691,000	0													161,700,000
Line of Credit Facility, Unsecured, Annual Facility Fee																												annual facility fee of 15 basis points
Line of Credit Facility, Unsecured, Interest Rate Description																											variable interest rate of LIBOR plus 55 basis points
Line of Credit Facility, Revolving, Maximum Borrowing Capacity																											600,000,000	600,000,000			113,800,000
Line of Credit Facility, Revolving, Expiration Date																												February 2012		February 2011
Long-term Debt, Amount Outstanding	1,970,145,000	2,094,469,000								34,779,000	395,700,000	1,539,666,000	[1]	1,940,145,000	2,084,469,000	1,509,666,000	1,671,129,000	417,712,000	402,151,000		12,767,000	11,189,000
Unsecured Line of Credit, Amount Outstanding			30,000,000		10,000,000		30,000,000		10,000,000																	0	30,000,000	30,000,000	10,000,000
Maturities of Long-term Debt [Abstract]
2011	31,675,000									2,825,000	8,850,000	20,000,000	[1]
2012	228,213,000									5,836,000	0	222,377,000	[1]
2013	22,105,000									5,763,000	16,342,000	0	[1]
2014	176,102,000									5,174,000	20,928,000	150,000,000	[1]
2015	400,096,000									3,783,000	46,313,000	350,000,000	[1]
Beyond 5 Years	1,113,636,000									11,398,000	302,238,000	800,000,000	[1]
Unamortized debt discounts (premiums)	$ (1,682,000)									$ 0	$ 1,029,000	$ (2,711,000)	[1]

[1] Includes unsecured public debt and the Line. The Line is included in 2012 maturities and matures in February 2012.