Notes Payable and Unsecured Credit Facilities (Details) - USD ($)		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Aug. 05, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Document Period End Date		Jun. 30, 2015
Line of Credit Facility, Expiration Date		May 13, 2019
Total debt outstanding		$ 1,988,700,000			$ 2,021,357,000
Document Fiscal Year Focus		2015
Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		5.40%
Total notes payable		$ 438,683,000			518,993,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding		438,683,000			518,993,000
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Total notes payable		1,868,700,000			1,946,357,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding		$ 1,868,700,000			1,946,357,000
Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		1.10%
Total notes payable		$ 32,238,000			29,839,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding		32,238,000			29,839,000
Fixed Rate Unsecured Loans [Member]
Debt Instrument [Line Items]
Total notes payable		1,397,779,000			1,397,525,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Amount Outstanding		1,397,779,000			1,397,525,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities		$ 120,000,000			75,000,000
Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
Line of Credit Facility, Commitment Fee Percentage		9250.00%
Debt instrument, Facility Fee basis points		$ 15
Proceeds from Lines of Credit		45,000,000
Borrowing capacity		800,000,000
Line of Credit [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities		45,000,000			0
Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities		75,000,000			$ 75,000,000
Parent Company [Member]
Debt Instrument [Line Items]
Debt repaid		0	$ 150,000,000
5.650% & 5.360% maturing in 2015 [Member]
Debt Instrument [Line Items]
Repayments of Debt		76,000,000
$165 mill Term Loan LIBOR plus 1.15, .20% unused fee [Member] | Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Borrowing capacity		67.40
Consolidated Entities [Member] | 2.196 Fixed rate with interest rate swap LIBOR 1.5% Maturing 10/16/20 [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Proceeds from Construction Loans Payable		2,400,000
Scheduled Principal Payments [Member]
Debt Instrument [Line Items]
Total notes payable		38,244,000
Maturities of Long-term Debt [Abstract]
2015		3,175,000
2016		6,135,000
2017		5,399,000
2018		4,453,000
2019		3,443,000
Beyond 5 Years		15,639,000
Unamortized debt (discounts) premiums, net		0
Long-term Debt, Amount Outstanding		38,244,000
Mortgage Loan Maturities [Member]
Debt Instrument [Line Items]
Total notes payable		432,677,000
Maturities of Long-term Debt [Abstract]
2015		141,000
2016		41,442,000
2017		116,425,000
2018		57,358,000
2019		106,000,000
Beyond 5 Years		105,447,000
Unamortized debt (discounts) premiums, net		5,864,000
Long-term Debt, Amount Outstanding		432,677,000
Unsecured Maturities [Member]
Debt Instrument [Line Items]
Total notes payable	[1]	1,517,779,000
Maturities of Long-term Debt [Abstract]
2015	[1]	350,000,000
2016	[1]	0
2017	[1]	400,000,000
2018	[1]	0
2019	[1]	120,000,000
Beyond 5 Years	[1]	650,000,000
Unamortized debt (discounts) premiums, net	[1]	(2,221,000)
Long-term Debt, Amount Outstanding	[1]	1,517,779,000
Long-term Debt [Member]
Debt Instrument [Line Items]
Total notes payable		1,988,700,000
Maturities of Long-term Debt [Abstract]
2015		353,316,000
2016		47,577,000
2017		521,824,000
2018		61,811,000
2019		229,443,000
Beyond 5 Years		771,086,000
Unamortized debt (discounts) premiums, net		3,643,000
Long-term Debt, Amount Outstanding		$ 1,988,700,000
Subsequent Event [Member]
Maturities of Long-term Debt [Abstract]
Repayment of Unsecured Debt, Subsequent to Period End Date	[1]			$ 350,000,000

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