Schedule of Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 3,840,268	$ 3,594,977
Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	526,048	520,193
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt	3,276,888	2,971,715
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	563,380	623,262
Long-term Debt	2,623,209	2,325,656
Line of Credit [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	0	60,000
Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities	563,380	563,262
London Interbank Offered Rate (LIBOR) [Member] | Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 127,631	$ 125,866
Contractual Rate [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.64%
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	4.10%
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	2.81%
Effective Rate [Member] | Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.43%
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	4.50%
Effective Rate [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.50%