Notes Payable and Unsecured Credit Facilities (Details) - USD ($) $ in Thousands			9 Months Ended
	Jan. 05, 2022	Jul. 07, 2016	Sep. 30, 2016	Sep. 30, 2015	Jul. 13, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Document Period End Date			Sep. 30, 2016
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year			$ 1,428
2017			122,805
Long-term Debt, Maturities, Repayments of Principal in Year Three			62,184
2019			109,753
2020			238,313
Beyond 5 Years			1,097,004
Unamortized debt (discounts) premiums, net			(3,866)
Long-term Debt			1,627,621			$ 1,864,285
Mortgages [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt			418,358			475,214
Notes Payable to Banks [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt			1,364,200			1,699,771
Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 300,000
Total credit facilities			263,421			164,514
Maturities of Long-term Debt [Abstract]
Long-term Debt			891,867			1,190,403
Debt Instrument, Interest Rate, Stated Percentage					5.875%
Notice of Repayment of Unsecured Debt Subsequent to Period End Date, Make-Whole Premium Costs					$ 13,200
Line of Credit [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities			0			0
Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Increase in Borrowing Capacity		$ 100,000
Line of Credit Facility, Maximum Borrowing Capacity		$ 265,000
Line of Credit Facility, Expiration Date	Jan. 05, 2022
Line of Credit Facility, Interest Rate Description		LIBOR plus a ratings based margin of 0.95%
Line of Credit Facility, Interest Rate at Period End		2.00%
Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities			263,421			164,514
Parent Company [Member]
Debt Instrument [Line Items]
Repayments of Notes Payable			41,584	$ 76,027
Debt repaid			$ 300,000	$ 350,000
Construction Loans [Member] | Variable Income Interest Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate Terms			one month LIBOR plus 150 basis points
Fellsway $28.1M, Swap fixed 3.696% [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage			3.696%
Blackrock $20M, Swap fixed 2.803% [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage			2.803%
Scheduled Principal Payments [Member] | Mortgages [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year			$ 1,428
2017			5,507
Long-term Debt, Maturities, Repayments of Principal in Year Three			4,826
2019			3,753
2020			4,091
Beyond 5 Years			11,933
Unamortized debt (discounts) premiums, net			0
Long-term Debt			31,538
Mortgage Loan Maturities [Member] | Mortgages [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year			0
2017			117,298
Long-term Debt, Maturities, Repayments of Principal in Year Three			57,358
2019			106,000
2020			84,222
Beyond 5 Years			70,071
Unamortized debt (discounts) premiums, net			5,846
Long-term Debt			440,795
Unsecured Maturities [Member] | Unsecured Debt [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year			0
2017			0
Long-term Debt, Maturities, Repayments of Principal in Year Three			0
2019			0
2020			150,000
Beyond 5 Years			1,015,000
Unamortized debt (discounts) premiums, net			(9,712)
Long-term Debt			1,155,288
London Interbank Offered Rate (LIBOR) [Member] | Mortgages [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Long-term Debt			20,000
Maturities of Long-term Debt [Abstract]
Long-term Debt			53,975			$ 34,154
Derivative @ 2.196% 28.1M [Member]
Debt Instrument [Line Items]
Derivative, Notional Amount			28,100
Derivative @ 1.30250% 20.0M [Member]
Debt Instrument [Line Items]
Derivative, Notional Amount			$ 20,000