Notes Payable and Unsecured Credit Facilities (Details) (USD $)								3 Months Ended											0 Months Ended	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Scheduled Principal Payments [Member]	Mar. 31, 2013 Mortgage Loan Maturities [Member]	Mar. 31, 2013 Unsecured Maturities [Member]		Mar. 31, 2013 Long-term Debt [Member]	Mar. 31, 2013 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Mar. 31, 2013 Notes Payable to Banks [Member]	Dec. 31, 2012 Notes Payable to Banks [Member]	Mar. 31, 2013 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2012 Fixed Rate Mortgage Loans [Member]	Mar. 31, 2013 Variable Rate Mortgage Loans [Member]	Dec. 31, 2012 Variable Rate Mortgage Loans [Member]	Mar. 31, 2013 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2012 Fixed Rate Unsecured Loans [Member]	Mar. 31, 2013 Unsecured Credit Facilities [Member]	Dec. 31, 2012 Unsecured Credit Facilities [Member]	Mar. 04, 2013 Consolidated Entities [Member] Mortgages [Member] 3.3% Mortgage Maturing April 1, 2020 [Member]	Mar. 31, 2013 Consolidated Entities [Member] Mortgages [Member] 3.3% Mortgage Maturing April 1, 2020 [Member]	Mar. 31, 2013 Hilltop Village [Member] Consolidated Entities [Member] Mortgages [Member]	Mar. 31, 2013 Hilltop Village [Member] Consolidated Entities [Member] Mortgages [Member] Five Point Six Percent Mortgage Maturing April 6, 2016 [Member]	Mar. 20, 2013 Hilltop Village [Member] Consolidated Entities [Member] Mortgages [Member] Five Point Six Percent Mortgage Maturing April 6, 2016 [Member]
Debt Instrument [Line Items]
Unsecured Line of Credit, Amount Outstanding								$ 25,000,000
Line of Credit Facility, Maximum Borrowing Capacity								800,000,000
Debt Instrument, Increase, Additional Borrowings																			8,250,000
Mortgage loans assumed for the acquisition of real estate																					7,500,000	7,500,000
Debt Instrument, Interest Rate, Stated Percentage																			3.30%				5.57%
Debt Instrument, Maturity Date																				Apr. 01, 2020		Apr. 06, 2016
Long-term Debt, Amount Outstanding	1,930,443,000	1,941,891,000	49,013,000	438,390,000	1,443,040,000	[1]	1,930,443,000		1,785,443,000	1,771,891,000	475,443,000	461,914,000	11,960,000	12,041,000	1,298,040,000	1,297,936,000	145,000,000	170,000,000
Maturities of Long-term Debt [Abstract]
2013			5,791,000	16,317,000	0	[1]	22,108,000
2014			7,383,000	26,912,000	150,000,000	[1]	184,295,000
2015			5,747,000	62,435,000	350,000,000	[1]	418,182,000
2016			5,487,000	21,661,000	145,000,000	[1]	172,148,000
2017			4,584,000	84,484,000	400,000,000	[1]	489,068,000
Beyond 5 Years			20,021,000	220,993,000	400,000,000	[1]	641,014,000
Unamortized debt (discounts) premiums, net			0	5,588,000	(1,960,000)	[1]	3,628,000
Long-term Debt, Amount Outstanding	$ 1,930,443,000	$ 1,941,891,000	$ 49,013,000	$ 438,390,000	$ 1,443,040,000	[1]	$ 1,930,443,000		$ 1,785,443,000	$ 1,771,891,000	$ 475,443,000	$ 461,914,000	$ 11,960,000	$ 12,041,000	$ 1,298,040,000	$ 1,297,936,000	$ 145,000,000	$ 170,000,000

[1] Includes unsecured public debt and unsecured credit facilities balances outstanding as of March 31, 2013.