Debt (Narrative) (Details)	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) loan item	Dec. 31, 2017 USD ($) loan
Debt Instrument [Line Items]
Credit facility, amount outstanding	$ 5,350,000
Number of interest rate cap transactions \| item	3
Interest rate cap notional value	$ 150,000,000
Weighted average fixed annual rate	3.80%
$50.0 Million Term Loan Maturing August 1, 2021, And $100.0 Million Term Loan Maturing January, 1 2022 [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	$ 150,000,000	$ 150,000,000
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Weighted average fixed annual rate	4.10%
Senior Unsecured Notes [Member] \| July 2024 Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 100,000,000
Debt Instrument, Maturity Date	Jul. 01, 2024
Senior Unsecured Notes [Member] \| September 2022 Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 50,000,000
Debt Instrument, Maturity Date	Sep. 01, 2022
Senior Unsecured Notes [Member] \| July 2026 Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 50,000,000
Debt Instrument, Maturity Date	Jul. 01, 2026
Senior Unsecured Notes [Member] \| October 2027 Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 50,000,000
Debt Instrument, Maturity Date	Oct. 01, 2027
Mortgage Loans Payable [Member]
Debt Instrument [Line Items]
Number of mortgage loans payable \| loan	3	3
Secured loan	$ 64,000,000	$ 64,800,000
Weighted average fixed annual rate	4.00%	4.00%
Total net investment book value of properties securing the debt	$ 154,000,000	$ 153,700,000
Mortgage Loans Payable [Member] \| Minimum [Member[
Debt Instrument [Line Items]
Mortgage loans maturity, period	2019
Mortgage Loans Payable [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Mortgage loans maturity, period	2021
Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum	$ 600,000,000
Borrowing Base Properties Percentage	60.00%
Effective rate	1.35%
LIBOR margin	1.25%
Credit facility, amount outstanding	$ 5,400,000	$ 0
Facility [Member] \| August 2020 Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Aug. 01, 2020
Credit facility, current	$ 200,000,000
Facility [Member] \| August 2021 Term Loan [Member]
Debt Instrument [Line Items]
Credit facility, current	$ 50,000,000
Maturity date, line of credit	Aug. 01, 2021
Facility [Member] \| January 2022 Term Loan [Member]
Debt Instrument [Line Items]
Credit facility, current	$ 100,000,000
Maturity date, line of credit	Jan. 01, 2022
Facility [Member] \| $50.0 Million Term Loan Maturing August 1, 2021, And $100.0 Million Term Loan Maturing January, 1 2022 [Member]
Debt Instrument [Line Items]
Effective rate	1.30%
Facility [Member] \| Federal Funds Rate [Member]
Debt Instrument [Line Items]
Effective rate	0.50%
Facility [Member] \| Minimum [Member[
Debt Instrument [Line Items]
LIBOR margin	1.35%
Unused facility fee rate	0.20%
Facility [Member] \| Minimum [Member[ \| $50.0 Million Term Loan Maturing August 1, 2021, And $100.0 Million Term Loan Maturing January, 1 2022 [Member]
Debt Instrument [Line Items]
LIBOR margin	1.30%
Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
LIBOR margin	1.90%
Unused facility fee rate	0.25%
Facility [Member] \| Maximum [Member] \| $50.0 Million Term Loan Maturing August 1, 2021, And $100.0 Million Term Loan Maturing January, 1 2022 [Member]
Debt Instrument [Line Items]
LIBOR margin	1.85%
Interest Rate Cap [Member]
Debt Instrument [Line Items]
Number of interest rate cap transactions \| item	3
Interest rate cap notional value	$ 150,000,000

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Borrowing Base Properties Percentage

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Investment under secured debt book value net.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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Weighted average interest rate of debt outstanding.

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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Section S99
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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Date the credit facility terminates, in CCYY-MM-DD format.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

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- Definition

Indicates the number of mortgages under each classification.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Carrying value as of the balance sheet date of the portion of long-term, collateralized debt obligations due within one year or 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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