Mortgage Loans Payable and Credit Facility (Summary of Derivative Financial Instruments Held) (Details) - Interest Rate Swap [Member]	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
Derivatives Fair Value [Line Items]
Count \| contract		1
Fair value \| $		$ 511,000
Maturity dates		2021
Cash Flow Hedging, Count 7 [Member]
Derivatives Fair Value [Line Items]
Count \| contract	9
Fair value \| $	$ 15,755,000
Cash Flow Hedging, Count 7 [Member] \| Minimum [Member]
Derivatives Fair Value [Line Items]
Maturity dates	2019
Cash Flow Hedging, Count 7 [Member] \| Maximum [Member]
Derivatives Fair Value [Line Items]
Maturity dates	2025
Cash Flow Hedging, Count 6 [Member]
Derivatives Fair Value [Line Items]
Count \| contract		6
Fair value \| $		$ 6,394,000
Cash Flow Hedging, Count 6 [Member] \| Minimum [Member]
Derivatives Fair Value [Line Items]
Maturity dates		2019
Cash Flow Hedging, Count 6 [Member] \| Maximum [Member]
Derivatives Fair Value [Line Items]
Maturity dates		2024

Derivative maturity year.

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

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- Definition

Number of interest rate derivative instruments held by the entity at the reporting date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

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