Notes Payable and Unsecured Credit Facilities (Details) (USD $)			6 Months Ended						6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Term Loan - Unsecured [Member]	Jun. 30, 2012 675Notes [Member]	Jun. 30, 2012 Scheduled Principal Payments [Member]	Jun. 30, 2012 Mortgage Loan Maturities [Member]	Jun. 30, 2012 Unsecured Maturities [Member]		Jun. 30, 2012 Line of Credit Facility - Unsecured [Member]	Jun. 30, 2012 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Jun. 30, 2012 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2011 Fixed Rate Mortgage Loans [Member]	Jun. 30, 2012 Variable Rate Mortgage Loans [Member]	Dec. 31, 2011 Variable Rate Mortgage Loans [Member]	Jun. 30, 2012 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2011 Fixed Rate Unsecured Loans [Member]	Jun. 30, 2012 Notes Payable to Banks [Member]	Dec. 31, 2011 Notes Payable to Banks [Member]	Jun. 30, 2012 Unsecured Debt [Member] Term Loan - Unsecured [Member]
Debt Instrument [Line Items]
Repayments of Unsecured Debt				$ 192,400,000
Debt Instrument, Term				10 years
Debt Instrument, Interest Rate Terms				6.75%
Proceeds from Lines of Credit									115,000,000
Line of Credit Facility, Maximum Borrowing Capacity										600,000,000
Proceeds from Unsecured Notes Payable			150,000,000
Debt Instrument, Face Amount																			250,000,000
Debt Instrument, Maturity Date				Jan. 15, 2012
Long-term Debt, Amount Outstanding	2,063,299,000	1,982,440,000			54,280,000	406,291,000	1,602,728,000	[1]			448,037,000	439,880,000	12,534,000	12,665,000	1,297,728,000	1,489,895,000	1,758,299,000	1,942,440,000
Unsecured Line of Credit, Amount Outstanding	305,000,000	40,000,000
Maturities of Long-term Debt [Abstract]
2012	3,883,000				3,883,000	0	0	[1]
2013	23,928,000				7,595,000	16,333,000	0	[1]
2014	185,491,000				7,091,000	28,400,000	150,000,000	[1]
2015	557,103,000				5,618,000	46,485,000	505,000,000	[1]
2016	169,648,000				5,487,000	14,161,000	150,000,000	[1]
Beyond 5 Years	1,123,687,000				24,606,000	299,081,000	800,000,000	[1]
Unamortized debt (discounts) premiums, net	(441,000)				0	1,831,000	(2,272,000)	[1]
Long-term Debt, Amount Outstanding	$ 2,063,299,000	$ 1,982,440,000			$ 54,280,000	$ 406,291,000	$ 1,602,728,000	[1]			$ 448,037,000	$ 439,880,000	$ 12,534,000	$ 12,665,000	$ 1,297,728,000	$ 1,489,895,000	$ 1,758,299,000	$ 1,942,440,000

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