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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
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-Article 12
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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
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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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Average fixed interest rate related to the group of interest rate derivatives.

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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 ($)		6 Months Ended	12 Months Ended
	Feb. 20, 2018	Jun. 30, 2019	Dec. 31, 2018	Mar. 31, 2019
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 300,000,000.0
Long-term debt outstanding		$ 1,695,702,000	$ 1,560,333,000
Letters of credit, outstanding amount		31,300,000	19,700,000
Core Portfolio
Debt Instrument [Line Items]
Derivative, notional amount		587,574,000
Fund Portfolio \| Fund II
Debt Instrument [Line Items]
Long-term debt outstanding		23,598,000
Derivative, notional amount		42,501,000
Fund Portfolio \| Fund IV
Debt Instrument [Line Items]
Derivative, notional amount		202,114,000
Fund Portfolio \| Fund V
Debt Instrument [Line Items]
Derivative, notional amount		156,900,000
Interest Rate Swaps
Debt Instrument [Line Items]
Derivative, notional amount		$ 156,000,000.0
Derivative weighted average interest rate		2.43%
Unsecured Debt \| Fund IV \| Letter of Credit
Debt Instrument [Line Items]
Letters of credit, outstanding amount		$ 11,000,000.0	7,400,000
Unsecured Debt \| Core Portfolio
Debt Instrument [Line Items]
Long-term debt outstanding			383,000
Unsecured Notes Payable \| Unsecured Debt
Debt Instrument [Line Items]
Debt available balance		15,200,000	62,300,000
Unsecured Notes Payable \| Unsecured Debt \| Fund Portfolio \| Fund II
Debt Instrument [Line Items]
Long-term debt outstanding		40,000,000	40,000,000
$300 Million Term Loan \| Unsecured Debt \| Core Portfolio
Debt Instrument [Line Items]
Maximum borrowing capacity	300,000,000.0
$350 Million Term Loan \| Unsecured Debt \| Core Portfolio
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 350,000,000.0
Long-term debt outstanding		350,000,000.0	350,000,000.0
$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		40,000,000.0
Long-term line of credit, noncurrent		40,000,000.0	40,000,000.0
Remaining borrowing capacity		0.0	0.0
Bridge facility \| Unsecured Debt \| Fund IV
Debt Instrument [Line Items]
Maximum borrowing capacity		80,200,000		$ 41,800,000
Long-term line of credit, noncurrent		79,200,000	40,800,000
Remaining borrowing capacity		1,000,000.0	1,000,000.0
Subscription Line \| Unsecured Debt \| Fund IV
Debt Instrument [Line Items]
Maximum borrowing capacity		27,000,000.0		$ 15,000,000.0
Long-term line of credit, noncurrent		8,400,000	0
Remaining borrowing capacity		7,600,000	7,600,000
Subscription Line \| Unsecured Debt \| Fund V
Debt Instrument [Line Items]
Maximum borrowing capacity		150,000,000.0
Long-term line of credit, noncurrent		143,400,000
Remaining borrowing capacity		6,600,000
Subscription Line \| Unsecured Debt \| Fund Portfolio \| Fund V
Debt Instrument [Line Items]
Long-term debt outstanding		$ 143,400,000	102,800,000
Remaining borrowing capacity			$ 47,200,000
Subscription Line \| Unsecured Debt \| Fund Portfolio \| LIBOR \| Fund V
Debt Instrument [Line Items]
Basis spread on variable rate		1.60%	1.60%