Notes Payable and Unsecured Credit Facilities (Details) (USD $) In Thousands, unless otherwise specified			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 675Notes [Member]	Mar. 31, 2012 Notes Payable to Banks [Member]	Dec. 31, 2011 Notes Payable to Banks [Member]	Mar. 31, 2012 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2011 Fixed Rate Mortgage Loans [Member]	Mar. 31, 2012 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2011 Fixed Rate Unsecured Loans [Member]	Mar. 31, 2012 Variable Rate Mortgage Loans [Member]	Dec. 31, 2011 Variable Rate Mortgage Loans [Member]	Mar. 31, 2012 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Mar. 31, 2011 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Mar. 31, 2012 Scheduled Principal Payments [Member]	Mar. 31, 2012 Mortgage Loan Maturities [Member]	Mar. 31, 2012 Unsecured Maturities [Member]
Debt Instrument [Line Items]
Debt Instrument, Term			10 years
Repayments of Unsecured Debt			$ 192,400
Debt Instrument, Interest Rate Terms			6.75%
Debt Instrument, Maturity Date			Jan. 15, 2012
Long-term Debt, Amount Outstanding	2,023,358	1,982,440		1,748,358	1,942,440	438,120	439,880	1,297,624	1,489,895	12,614	12,665			53,299	397,435	1,572,624	[1]
Unsecured Line of Credit, Amount Outstanding												275,000	40,000
Maturities of Long-term Debt [Abstract]
2012	5,306													5,306	0	0	[1]
2013	23,327													6,995	16,332	0	[1]
2014	184,964													6,481	28,483	150,000	[1]
2015	526,483													5,170	46,313	475,000	[1]
2016	169,018													4,857	14,161	150,000	[1]
Beyond 5 Years	1,112,537													24,490	288,047	800,000	[1]
Unamortized debt (discounts) premiums, net	1,723													0	4,099	(2,376)	[1]
Long-term Debt, Amount Outstanding	$ 2,023,358	$ 1,982,440		$ 1,748,358	$ 1,942,440	$ 438,120	$ 439,880	$ 1,297,624	$ 1,489,895	$ 12,614	$ 12,665			$ 53,299	$ 397,435	$ 1,572,624	[1]

[1] Includes unsecured public debt and unsecured credit facilities balances outstanding as of March 31, 2012.