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

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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
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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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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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Fixed interest rate related to the interest rate derivative.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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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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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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The carrying value as of the balance sheet date of the noncurrent portion 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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Debt - Unsecured Notes Payable - Additional Information (Details) - USD ($)		12 Months Ended
	Feb. 20, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 300,000,000
Long-term debt outstanding		$ 1,560,333,000	$ 1,438,386,000
Derivative, notional amount		69,670,000
Letters of credit, outstanding amount		19,700,000	19,700,000
Core Portfolio
Debt Instrument [Line Items]
Derivative, notional amount		432,200,000
Fund Portfolio \| Fund II
Debt Instrument [Line Items]
Derivative, notional amount		19,325,000
Fund Portfolio \| Fund IV
Debt Instrument [Line Items]
Derivative, notional amount		180,741,000
Fund Portfolio \| Fund V
Debt Instrument [Line Items]
Derivative, notional amount		86,570,000
Interest Rate Swaps
Debt Instrument [Line Items]
Derivative, notional amount		$ 50,000,000
Derivative fixed interest rate		2.80%
Unsecured Debt \| Fund IV \| Letter of Credit
Debt Instrument [Line Items]
Letters of credit, outstanding amount		$ 7,400,000	7,400,000
Unsecured Debt \| Core Portfolio
Debt Instrument [Line Items]
Long-term debt outstanding		350,000,000	300,000,000
Unsecured Notes Payable \| Unsecured Debt
Debt Instrument [Line Items]
Debt available balance		62,300,000	70,300,000
Unsecured Notes Payable \| Unsecured Debt \| Fund Portfolio \| Fund II
Debt Instrument [Line Items]
Long-term debt outstanding		40,000,000	31,500,000
$300 Million Term Loan \| Unsecured Debt \| Core Portfolio
Debt Instrument [Line Items]
Maximum borrowing capacity	300,000,000
$350 Million Term Loan \| Unsecured Debt \| Core Portfolio
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 350,000,000
Long-term debt outstanding		350,000,000	300,000,000
$350 Million Term Loan \| Unsecured Debt \| Core Portfolio \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
Term Loan Maturing In September 2020 \| Secured Debt \| Fund II
Debt Instrument [Line Items]
Maximum borrowing capacity		40,000,000
Long-term line of credit, noncurrent		40,000,000	31,500,000
Remaining borrowing capacity		0	8,500,000
Bridge facility \| Unsecured Debt \| Fund IV
Debt Instrument [Line Items]
Maximum borrowing capacity		41,800,000
Long-term line of credit, noncurrent		40,800,000	40,800,000
Remaining borrowing capacity		1,000,000	1,000,000
Subscription Line \| Unsecured Debt \| Fund IV
Debt Instrument [Line Items]
Maximum borrowing capacity		21,500,000
Long-term line of credit, noncurrent		0	0
Remaining borrowing capacity		14,100,000	14,100,000
Subscription Line \| Unsecured Debt \| Fund V
Debt Instrument [Line Items]
Maximum borrowing capacity		150,000,000
Long-term line of credit, noncurrent		102,800,000
Remaining borrowing capacity		47,200,000
Subscription Line \| Unsecured Debt \| Fund Portfolio \| Fund V
Debt Instrument [Line Items]
Long-term debt outstanding		$ 102,800,000	103,300,000
Remaining borrowing capacity			$ 46,700,000
Subscription Line \| Unsecured Debt \| Fund Portfolio \| LIBOR \| Fund V
Debt Instrument [Line Items]
Basis spread on variable rate		1.60%	1.60%