Notes Payable and Unsecured Credit Facilities (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	$ 1,821,412,000			$ 1,821,412,000			$ 1,982,440,000
Unsecured Line of Credit, Amount Outstanding	65,000,000			65,000,000			40,000,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding	1,821,412,000			1,821,412,000			1,982,440,000
675Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Term				10 years
Scheduled Principal Payments [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	52,418,000			52,418,000
Maturities of Long-term Debt [Abstract]
2012	2,022,000			2,022,000
2013	7,595,000			7,595,000
2014	7,091,000			7,091,000
2015	5,618,000			5,618,000
2016	5,487,000			5,487,000
Beyond 5 Years	24,605,000			24,605,000
Unamortized debt (discounts) premiums, net	0			0
Long-term Debt, Amount Outstanding	52,418,000			52,418,000
Mortgage Loan Maturities [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	406,162,000			406,162,000
Maturities of Long-term Debt [Abstract]
2012	0			0
2013	16,330,000			16,330,000
2014	27,183,000			27,183,000
2015	46,485,000			46,485,000
2016	14,161,000			14,161,000
Beyond 5 Years	297,009,000			297,009,000
Unamortized debt (discounts) premiums, net	4,994,000			4,994,000
Long-term Debt, Amount Outstanding	406,162,000			406,162,000
Unsecured Maturities [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	1,362,832,000	[1]		1,362,832,000	[1]
Maturities of Long-term Debt [Abstract]
2012	0	[1]		0	[1]
2013	0	[1]		0	[1]
2014	150,000,000	[1]		150,000,000	[1]
2015	350,000,000	[1]		350,000,000	[1]
2016	65,000,000	[1]		65,000,000	[1]
Beyond 5 Years	800,000,000	[1]		800,000,000	[1]
Unamortized debt (discounts) premiums, net	(2,168,000)	[1]		(2,168,000)	[1]
Long-term Debt, Amount Outstanding	1,362,832,000	[1]		1,362,832,000	[1]
Long-term Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	1,821,412,000			1,821,412,000
Maturities of Long-term Debt [Abstract]
2012	2,022,000			2,022,000
2013	23,925,000			23,925,000
2014	184,274,000			184,274,000
2015	402,103,000			402,103,000
2016	84,648,000			84,648,000
Beyond 5 Years	1,121,614,000			1,121,614,000
Unamortized debt (discounts) premiums, net	2,826,000			2,826,000
Long-term Debt, Amount Outstanding	1,821,412,000			1,821,412,000
Line of Credit [Member] | Line of Credit Facility - Unsecured [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit				25,000,000
Line of Credit Facility, Maximum Borrowing Capacity	800,000,000			800,000,000
Line of Credit Facility, Interest Rate at Period End	11750.00%			11750.00%
Line of Credit Facility, Maximum Increase Allowed to Borrowing Capacity	200,000,000			200,000,000
Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	446,358,000			446,358,000			439,880,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding	446,358,000			446,358,000			439,880,000
Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	12,222,000			12,222,000			12,665,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding	12,222,000			12,222,000			12,665,000
Fixed Rate Unsecured Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	1,297,832,000			1,297,832,000			1,489,895,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding	1,297,832,000			1,297,832,000			1,489,895,000
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt, Amount Outstanding	1,756,412,000			1,756,412,000			1,942,440,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding	1,756,412,000			1,756,412,000			1,942,440,000
Unsecured Debt [Member] | Term Loan - Unsecured [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Debt			150,000,000
Unsecured Debt [Member] | 675Notes [Member]
Debt Instrument [Line Items]
Repayments of Unsecured Debt				192,400,000
Debt Instrument, Interest Rate Terms	6.75%			6.75%
Debt Instrument, Maturity Date				Jan. 15, 2012
Parent Company [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit				535,000,000		385,000,000
Mortgage loans assumed for the acquisition of real estate				12,810,000		31,186,000
Unsecured Line of Credit, Amount Outstanding	65,000,000			65,000,000			40,000,000
Maturities of Long-term Debt [Abstract]
Gains (Losses) on Extinguishment of Debt	(852,000)		0	(852,000)		0
Parent Company [Member] | Term Loan - Unsecured [Member]
Maturities of Long-term Debt [Abstract]
Gains (Losses) on Extinguishment of Debt			$ 852,000

[1] Includes unsecured public debt and unsecured credit facilities balances outstanding as of September 30, 2012.