Notes and Bonds Payable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Notes and bonds payable	$ 1,431,494	$ 1,403,692
Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member]
Debt Instrument [Line Items]
Notes and bonds payable	206,000	85,000
Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member]
Debt Instrument [Line Items]
Notes and bonds payable	$ 200,000	200,000
Contractual interest rates, basis spread on variable rate	1.20%
Mortgage notes payable [Member]
Debt Instrument [Line Items]
Contractual interest rates, minimum	4.15%
Contractual interest rates, maximum	7.60%
Mortgage notes payable [Member] | 3.88% Unsecured Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Notes and bonds payable	$ 249,804	0
Contractual interest rates	3.88%
Mortgage notes payable [Member] | 4.15%-7.63% Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Notes and bonds payable	$ 129,087	$ 173,267
Contractual interest rates, minimum	4.15%
Contractual interest rates, maximum	7.63%
LIBOR [Member] | Line of Credit [Member] | Unsecured Credit Facility due 2017 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate	1.15%
LIBOR [Member] | Medium-term Notes [Member] | Unsecured Term Loan Due 2019 [Member]
Debt Instrument [Line Items]
Contractual interest rates, basis spread on variable rate	1.20%