Notes and Bonds Payable (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Notes and bonds payable	$ 1,335,732	$ 1,283,880
Line of Credit | Unsecured Credit Facility
Debt Instrument [Line Items]
Notes and bonds payable	$ 235,000	189,000
Effective interest rate (percent)	3.09%
Medium-term Notes | Unsecured Term Loan Facility, net of issuance costs (1)
Debt Instrument [Line Items]
Notes and bonds payable	$ 149,081	148,994
Effective interest rate (percent)	3.33%
Notional amount	$ 50,000
Derivative, fixed interest rate (percent)	2.46%
Basis spread on variable rate (percent)	1.10%
Senior Notes | Senior Notes due 2023, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable	$ 247,909	247,703
Effective interest rate (percent)	3.95%
Senior Notes | Senior Notes due 2025, net of discount and issuance costs (2)
Debt Instrument [Line Items]
Notes and bonds payable	$ 248,160	248,044
Effective interest rate (percent)	4.08%
Senior Notes | Senior Notes due 2028, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable	$ 294,977	294,757
Effective interest rate (percent)	3.84%
Mortgages | Mortgage notes payable, net of discounts and issuance costs and including premiums
Debt Instrument [Line Items]
Notes and bonds payable	$ 160,605	$ 155,382
Effective interest rate (percent)	4.83%
Interest Rate Swap | Medium-term Notes | Unsecured Term Loan Facility, net of issuance costs (1)
Debt Instrument [Line Items]
Notional amount	$ 25,000
Derivative, fixed interest rate (percent)	2.18%
Interest Rate Swap | Senior Notes | Senior Notes due 2025, net of discount and issuance costs (2)
Debt Instrument [Line Items]
Loss on forward starting interest rate swap agreement recognized in OCI	$ 1,700