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Notes Payable and Unsecured Credit Facilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Repayments of Notes Payable	$ 11,422
Maturities of Long-term Debt [Abstract]
2017	84,335
2018	151,457
2019	230,653
2020	544,518
2021	298,194
Beyond 5 Years	2,111,306
Unamortized debt premium/(discount) and issuance costs	(13,237)
Long-term Debt	3,407,226		$ 1,642,420
Fixed Rate Mortgage Loans [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt	607,173		384,786
Notes Payable to Banks [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt	2,749,202		1,363,925
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	658,024		278,495
Maturities of Long-term Debt [Abstract]
Long-term Debt	2,025,705		892,170
Line of Credit [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	95,000		15,000
Term Loan [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	563,024		263,495
Parent Company [Member]
Debt Instrument [Line Items]
Repayments of Notes Payable	$ 11,422	$ 27,281
Fellsway $28.1M, Swap fixed 3.696% [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	3.70%
Blackrock $20M, Swap fixed 2.803% [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	2.80%
Scheduled Principal Payments [Member] \| Mortgages [Member]
Maturities of Long-term Debt [Abstract]
2017	$ 8,824
2018	11,481
2019	11,251
2020	10,107
2021	9,193
Beyond 5 Years	41,308
Unamortized debt premium/(discount) and issuance costs	0
Long-term Debt	92,164
Mortgage Loan Maturities [Member] \| Mortgages [Member]
Maturities of Long-term Debt [Abstract]
2017	75,511
2018	139,976
2019	124,402
2020	84,411
2021	39,001
Beyond 5 Years	154,998
Unamortized debt premium/(discount) and issuance costs	13,035
Long-term Debt	631,334
Unsecured Maturities [Member] \| Unsecured Debt [Member]
Maturities of Long-term Debt [Abstract]
2017	0
2018	0
2019	95,000
2020	450,000
2021	250,000
Beyond 5 Years	1,915,000
Unamortized debt premium/(discount) and issuance costs	(26,272)
Long-term Debt	2,683,728
London Interbank Offered Rate (LIBOR) [Member] \| Variable Rate Mortgage Loans [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt	116,324		$ 86,969
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
Contractual Rate [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	1.80%
Contractual Rate [Member] \| Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.60%
Contractual Rate [Member] \| Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.13%
Contractual Rate [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.10%
Contractual Rate [Member] \| Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.40%
Effective Rate [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	1.90%
Effective Rate [Member] \| Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.70%
Effective Rate [Member] \| Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.29%
Effective Rate [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.60%
Effective Rate [Member] \| Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.40%