Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 3,717,208	$ 3,594,977
Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	406,072	520,193
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt	3,008,592	2,971,715
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	708,616	623,262
Long-term Debt	2,474,724	2,325,656
Line of Credit [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	145,000	60,000
Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	563,616	563,262
London Interbank Offered Rate (LIBOR) [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 127,796	$ 125,866
Contractual Rate [Member] | Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.80%
Contractual Rate [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.30%
Contractual Rate [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.00%
Contractual Rate [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.90%
Contractual Rate [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.40%
Effective Rate [Member] | Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.30%
Effective Rate [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.60%
Effective Rate [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.40%
Effective Rate [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.10%
Effective Rate [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.50%