Debt - Mortgage Payable - Additional Information (Details)	12 Months Ended
Dec. 31, 2018 USD ($) property loan derivative	Dec. 31, 2017 USD ($) property
Debt Instrument [Line Items]
Amount drawn on loan	$ 1,017,288,000	$ 909,174,000
Derivative, notional amount	69,670,000
Unsecured line of credit	0	41,500,000
Mortgages
Debt Instrument [Line Items]
Amount drawn on loan	$ 1,017,288,000	$ 909,174,000
Number of properties collateralized \| property	43	42
Interest Rate Swaps
Debt Instrument [Line Items]
Derivative, notional amount	$ 50,000,000
Derivative fixed interest rate	2.80%
Interest Rate Swaps \| Mortgages
Debt Instrument [Line Items]
Number of interest rate derivatives held \| derivative	4
Derivative, notional amount	$ 136,600,000
Derivative fixed interest rate	2.86%
Fund V \| Mortgages
Debt Instrument [Line Items]
Number of mortgage loans \| loan	4
Borrowings, amount	$ 109,500,000
Core Portfolio
Debt Instrument [Line Items]
Derivative, notional amount	432,200,000
Core Portfolio \| Maturing In 2028
Debt Instrument [Line Items]
Amount drawn on loan	$ 50,000,000
Core Portfolio \| Mortgages
Debt Instrument [Line Items]
Number of notes repaid \| loan	1
Repayments of debt	$ 40,400,000
Scheduled principal payment	6,700,000
Core Portfolio \| Mortgages \| Brandywine Portfolio
Debt Instrument [Line Items]
Debt default, amount	$ 26,300,000	$ 26,300,000
Interest rate, stated percentage	6.00%
Debt default interest	5.00%
Interest owner percentage in property	22.00%
Core Portfolio \| Mortgages \| Maturing In 2028
Debt Instrument [Line Items]
Borrowings, amount	$ 73,500,000
Fund III \| Construction Loans
Debt Instrument [Line Items]
Unsecured line of credit	$ 24,600,000
Fund IV \| Mortgages
Debt Instrument [Line Items]
Number of notes repaid \| loan	3
Fund IV \| Mortgages One
Debt Instrument [Line Items]
Repayments of debt	$ 27,200,000
LIBOR \| Fund V \| Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	1.99%
LIBOR \| Core Portfolio \| Mortgages
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%
LIBOR \| Core Portfolio \| Mortgages \| Maturing In 2028
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
LIBOR \| Fund IV \| Mortgages One
Debt Instrument [Line Items]
Basis spread on variable rate	2.81%

Debt instrument, default interest.

No definition available.

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- Definition

Mortgage debt, interest owner percentage in property.

+ References

No definition available.

+ Details

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- Definition

Mortgage loans on real estate, number of notes repaid.

+ References

No definition available.

+ Details

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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

Number of mortgage loans within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Footnote 3
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Footnote 3))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-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

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

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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- Definition

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

+ References

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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

Amount of the required periodic payments applied to principal.

+ References

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- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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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.

+ References

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Balance Type:	credit
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- Definition

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

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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.

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- Definition

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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