Debt - Mortgage Payable (Details)	12 Months Ended
Dec. 31, 2017 USD ($) property derivative loan	Dec. 31, 2016 USD ($) property
Debt Instrument [Line Items]
Number of interest rate derivatives held \| derivative	8
Mortgages
Debt Instrument [Line Items]
Number of mortgage loans \| loan	11
Borrowings, amount	$ 162,900,000.0
Number of properties collateralized \| property	42	39
Number of notes repaid \| loan	13
Repayments of debt	$ 280,800,000
Annual principal payment	1,000,000
Interest Rate Swaps \| Mortgages
Debt Instrument [Line Items]
Derivative, notional amount	$ 73,300,000
Weighted average interest rate	2.11%
Core Portfolio
Debt Instrument [Line Items]
Derivative, notional amount	$ 397,607,000
Brandywine Portfolio \| Core Portfolio \| Mortgages
Debt Instrument [Line Items]
Interest rate, stated percentage	5.99%
Debt default, amount	$ 26,300,000	$ 26,300,000
Debt default interest	5.00%
Interest owner percentage in property	22.00%
Lawsuit damages sought, value	$ 32,100,000
LIBOR \| Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	3.47%
Interest rate, stated percentage	3.90%

Debt Instrument, Default Interest

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- Definition

Mortgage Debt, Interest Owner Percentage in Property

+ References

No definition available.

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- Definition

Mortgage Loans On Real Estate, Number Of Notes Repaid

+ References

No definition available.

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- Definition

Number of Properties Collateralized

+ References

No definition available.

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- Definition

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

+ References

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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- Definition

Amount of outstanding long-term debt or borrowing associated with any securities or credit agreement for which there has been a default in principal, interest, sinking fund, or redemption provisions, or any breach of covenant that existed at the end of the period and subsequently has not been cured.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(c))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

Amount of the total principal payments made during the annual reporting period.

+ References

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-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

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- Definition

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

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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.

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- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

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- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

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- Definition

Indicates the number of mortgages under each classification.

+ References

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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- Definition

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

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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- Definition

Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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