Mortgage Loans Payable and Credit Facility (Schedule of Debt Related to Continuing Operations) (Details) - USD ($)	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Mortgage loans payable	$ 126,453,000	$ 127,969,000
Unsecured revolving credit facility	104,500,000	$ 55,000,000
Continuing Operations [Member]
Debt Instrument [Line Items]
Total debt gross	630,998,000
Unamortized premium	472,000
Unamortized debt issuance costs	(3,055,000)
Total debt	$ 628,415,000
Weighted average contractual interest rate	3.50%
Continuing Operations [Member] \| Term Loan Two [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Weighted average contractual interest rate	3.60%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member]
Debt Instrument [Line Items]
Maturity date range, start	2021
Maturity date range, end	2026
Mortgage loans payable	$ 126,498,000
Weighted average contractual interest rate	4.40%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Two [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 28, 2021
Unsecured revolving credit facility	$ 75,000,000
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Three [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 28, 2022
Unsecured revolving credit facility	$ 50,000,000
Weighted average contractual interest rate	3.00%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Four [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2022
Unsecured revolving credit facility	$ 50,000,000
Weighted average contractual interest rate	2.80%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Five [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 30, 2023
Unsecured revolving credit facility	$ 100,000,000
Weighted average contractual interest rate	3.20%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Six [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2024
Unsecured revolving credit facility	$ 75,000,000
Weighted average contractual interest rate	3.30%
Continuing Operations [Member] \| Variable Rate Mortgage \| Revolving Credit Facility [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2021
Unsecured revolving credit facility	$ 104,500,000
Weighted average contractual interest rate	3.30%
Continuing Operations [Member] \| Variable Rate Mortgage \| Term Loan [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2022
Unsecured revolving credit facility	$ 50,000,000
Weighted average contractual interest rate	3.40%

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Debt instrument maturity year range start.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt premium.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Weighted average interest rate of long-term debt outstanding calculated at point in time.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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