Schedule of Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 3,831,500	$ 3,594,977
Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	516,149	520,193
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt	3,118,002	2,971,715
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	713,498	623,262
Long-term Debt	2,474,126	2,325,656
Line of Credit [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	150,000	60,000
Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	563,498	563,262
London Interbank Offered Rate (LIBOR) [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 127,727	$ 125,866
Contractual Rate [Member] | Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.10%
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	2.80%
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	4.40%
Effective Rate [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.50%
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	3.00%
Effective Rate [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.50%