Notes and Bonds Payable (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Notes and bonds payable	$ 1,278,662	$ 1,264,370
Line of Credit | Unsecured Credit Facility Due 2020
Debt Instrument [Line Items]
Notes and bonds payable	$ 122,000	107,000
Effective interest rate (percent)	1.98%
Medium-term Notes | Term Loan Due 2019
Debt Instrument [Line Items]
Notes and bonds payable	$ 149,550	149,491
Effective interest rate (percent)	2.18%
Senior Notes | Senior Notes due 2021, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable	$ 397,315	397,147
Effective interest rate (percent)	5.97%
Senior Notes | Senior Notes due 2023, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable	$ 247,397	247,296
Effective interest rate (percent)	3.95%
Senior Notes | Senior Notes due 2025, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable	$ 247,874	247,819
Effective interest rate (percent)	4.08%
Mortgages
Debt Instrument [Line Items]
Notes and bonds payable	$ 114,526	$ 115,617
Effective interest rate (percent)	5.15%