Notes Payable and Unsecured Credit Facilities (Details) (USD $)																							3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Scheduled Principal Payments [Member]	Mar. 31, 2014 Mortgage Loan Maturities [Member]	Mar. 31, 2014 Unsecured Maturities [Member]		Mar. 31, 2014 Long-term Debt [Member]	Mar. 31, 2014 Notes Payable to Banks [Member]	Dec. 31, 2013 Notes Payable to Banks [Member]	Mar. 31, 2014 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2013 Fixed Rate Mortgage Loans [Member]	Mar. 31, 2014 Variable Rate Mortgage Loans [Member]	Dec. 31, 2013 Variable Rate Mortgage Loans [Member]	Mar. 31, 2014 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2013 Fixed Rate Unsecured Loans [Member]	Mar. 31, 2014 Unsecured Debt [Member]	Dec. 31, 2013 Unsecured Debt [Member]	Mar. 31, 2014 Line of Credit [Member]	Mar. 31, 2014 Line of Credit [Member] Unsecured Debt [Member]	Dec. 31, 2013 Line of Credit [Member] Unsecured Debt [Member]	Mar. 31, 2014 Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2013 Term Loan [Member] Unsecured Debt [Member]	Mar. 31, 2014 Parent Company [Member]	Mar. 31, 2013 Parent Company [Member]
Debt Instrument [Line Items]
Mortgage loans assumed for the acquisition of real estate, net of premiums																							$ 78,049,000	$ 0
Total notes payable			44,476,000	512,758,000	1,443,456,000	[1]	2,000,690,000	1,855,690,000	1,779,697,000	520,134,000	444,245,000	37,100,000	37,100,000	1,298,456,000	1,298,352,000
Total credit facilities																145,000,000	75,000,000	70,000,000	70,000,000	0	75,000,000	75,000,000
Total debt outstanding	2,000,690,000	1,854,697,000
Borrowing capacity																		800,000,000
Debt repaid					150,000,000	[1]
Maturities of Long-term Debt [Abstract]
2014			5,741,000	15,538,000	150,000,000	[1]	171,279,000
2015			6,358,000	82,685,000	350,000,000	[1]	439,043,000
2016			5,867,000	41,421,000	145,000,000	[1]	192,288,000
2017			5,121,000	115,857,000	400,000,000	[1]	520,978,000
2018			4,165,000	57,358,000	0	[1]	61,523,000
Beyond 5 Years			17,224,000	190,301,000	400,000,000	[1]	607,525,000
Unamortized debt (discounts) premiums, net			0	9,598,000	(1,544,000)	[1]	8,054,000
Long-term Debt, Amount Outstanding			$ 44,476,000	$ 512,758,000	$ 1,443,456,000	[1]	$ 2,000,690,000	$ 1,855,690,000	$ 1,779,697,000	$ 520,134,000	$ 444,245,000	$ 37,100,000	$ 37,100,000	$ 1,298,456,000	$ 1,298,352,000

[1] Includes unsecured public debt and unsecured credit facilities.