Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Notes Payable and Unsecured Credit Facilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Maturities of Long-term Debt [Abstract]
2017	$ 5,372
2018	150,617
2019	24,164
2020	512,702
2021	300,424
Beyond 5 Years	2,529,856
Unamortized debt premium/(discount) and issuance costs	(15,109)
Long-term Debt	3,508,026		$ 1,642,420
Fixed Rate Mortgage Loans [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt	500,447		384,786
Notes Payable to Banks [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt	2,944,995		1,363,925
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	563,031		278,495
Maturities of Long-term Debt [Abstract]
Long-term Debt	2,325,463		892,170
Line of Credit [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	0		15,000
Term Loan [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	$ 563,031		263,495
Minimum [Member] \| Mortgages [Member]
Maturities of Long-term Debt [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Maximum [Member] \| Mortgages [Member]
Maturities of Long-term Debt [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	7.80%
Parent Company [Member]
Debt Instrument [Line Items]
Repayments of Notes Payable	$ 232,839	$ 41,584
Scheduled Principal Payments [Member] \| Mortgages [Member]
Maturities of Long-term Debt [Abstract]
2017	5,372
2018	10,641
2019	10,948
2020	11,122
2021	11,426
Beyond 5 Years	48,674
Unamortized debt premium/(discount) and issuance costs	0
Long-term Debt	98,183
Mortgage Loan Maturities [Member] \| Mortgages [Member]
Maturities of Long-term Debt [Abstract]
2017	0
2018	139,976
2019	13,216
2020	51,580
2021	38,998
Beyond 5 Years	266,182
Unamortized debt premium/(discount) and issuance costs	11,397
Long-term Debt	521,349
Unsecured Maturities [Member] \| Unsecured Debt [Member]
Maturities of Long-term Debt [Abstract]
2017	0
2018	0
2019	0
2020	450,000
2021	250,000
Beyond 5 Years	2,215,000
Unamortized debt premium/(discount) and issuance costs	(26,506)
Long-term Debt	2,888,494
London Interbank Offered Rate (LIBOR) [Member] \| Variable Rate Mortgage Loans [Member]
Maturities of Long-term Debt [Abstract]
Long-term Debt	119,085		$ 86,969
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	1.90%
Contractual Rate [Member] \| Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.90%
Contractual Rate [Member] \| Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.43%
Contractual Rate [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.20%
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	2.00%
Effective Rate [Member] \| Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.20%
Effective Rate [Member] \| Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.63%
Effective Rate [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.70%
Effective Rate [Member] \| Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.50%