Mortgage Loans Payable and Unsecured Credit Facilities (Schedule of Debt Related to Continuing Operations) (Details) - USD ($)		12 Months Ended
Sep. 08, 2017	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Mortgage loans payable		$ 47,315,000	$ 127,969,000
Total debt gross		628,370,000
Credit facilities		100,000,000	55,000,000
Unsecured revolving credit facility		100,000,000	55,000,000
Unamortized issuance costs		3,536,000
Total debt		624,834,000
Capital Lease Obligation [Member]
Debt Instrument [Line Items]
Total debt gross		5,696,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total debt gross		100,000,000
Term Loan Two [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 28, 2021
Credit facilities	$ 75,000,000
Unsecured revolving credit facility	$ 75,000,000
Term Loan Three [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 28, 2022
Credit facilities	$ 50,000,000
Unsecured revolving credit facility	$ 50,000,000
Term Loan Four [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2022
Credit facilities	$ 50,000,000
Unsecured revolving credit facility	$ 50,000,000
Term Loan Six [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2024
Credit facilities	$ 75,000,000
Unsecured revolving credit facility	$ 75,000,000
Continuing Operations [Member]
Debt Instrument [Line Items]
Total debt gross		628,370,000	582,994,000
Unamortized premium			536,000
Unamortized issuance costs		3,536,000	3,405,000
Total debt		$ 624,834,000	$ 580,125,000
Weighted average contractual interest rate		3.60%	3.40%
Continuing Operations [Member] \| Capital Lease Obligation [Member]
Debt Instrument [Line Items]
Maturity date		Sep. 30, 2050
Total debt gross		$ 5,696,000
Weighted average contractual interest rate		5.30%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member]
Debt Instrument [Line Items]
Maturity date		Jun. 30, 2026
Maturity date range, start			2021
Maturity date range, end			2026
Mortgage loans payable		$ 47,674,000	$ 127,994,000
Weighted average contractual interest rate		3.90%	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	Feb. 28, 2021
Credit facilities		$ 75,000,000	$ 75,000,000
Unsecured revolving credit facility		$ 75,000,000	$ 75,000,000
Weighted average contractual interest rate		3.60%	3.60%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Three [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date		Feb. 28, 2022	Feb. 28, 2022
Credit facilities		$ 50,000,000	$ 50,000,000
Unsecured revolving credit facility		$ 50,000,000	$ 50,000,000
Weighted average contractual interest rate		3.00%	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	Feb. 29, 2020
Credit facilities		$ 50,000,000	$ 50,000,000
Unsecured revolving credit facility		$ 50,000,000	$ 50,000,000
Weighted average contractual interest rate		2.80%	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	Apr. 30, 2023
Credit facilities		$ 100,000,000	$ 100,000,000
Unsecured revolving credit facility		$ 100,000,000	$ 100,000,000
Weighted average contractual interest rate		3.20%	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	Feb. 28, 2019
Credit facilities		$ 75,000,000	$ 75,000,000
Unsecured revolving credit facility		$ 75,000,000	$ 75,000,000
Weighted average contractual interest rate		3.30%	3.30%
Continuing Operations [Member] \| Fixed-Rate Mortgages [Member] \| Term Loan Seven [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date		Jul. 31, 2025
Credit facilities		$ 75,000,000
Unsecured revolving credit facility		$ 75,000,000
Weighted average contractual interest rate		4.60%
Continuing Operations [Member] \| Variable-Rate Mortgage [Member] \| Revolving Credit Facility [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date		Sep. 30, 2021	Sep. 30, 2021
Credit facilities		$ 100,000,000	$ 55,000,000
Unsecured revolving credit facility		$ 100,000,000	$ 55,000,000
Weighted average contractual interest rate		3.80%	2.80%
Continuing Operations [Member] \| Variable-Rate Mortgage [Member] \| Term Loan [Member] \| Unsecured credit facility [Member]
Debt Instrument [Line Items]
Maturity date		Sep. 30, 2022	Sep. 30, 2022
Credit facilities		$ 50,000,000	$ 50,000,000
Unsecured revolving credit facility		$ 50,000,000	$ 50,000,000
Weighted average contractual interest rate		3.80%	2.90%

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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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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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