Debt - Summary of Consolidated Indebtedness (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,664,142	$ 1,560,333
Net unamortized debt issuance costs	(9,463)	(10,541)
Unamortized fair market value of assumed debt	676	753
Mortgage and other notes payable, net	1,029,678	1,017,288
Unsecured notes payable, net	625,677	533,257
Total Indebtedness	1,655,355	1,550,545
Mortgages
Debt Instrument [Line Items]
Net unamortized debt issuance costs	(9,135)	(10,173)
Unamortized fair market value of assumed debt	676	753
Mortgage and other notes payable, net	$ 1,029,678	1,017,288
Mortgages \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Unsecured Debt
Debt Instrument [Line Items]
Net unamortized debt issuance costs	$ (328)	(368)
Unsecured notes payable, net	$ 625,677	533,257
Core Portfolio \| Minimum
Debt Instrument [Line Items]
Maturity Date	2019-10
Core Portfolio \| Maximum
Debt Instrument [Line Items]
Maturity Date	2020-04
Core Portfolio \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Core Portfolio \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.90%
Core Portfolio \| Unsecured Line of Credit \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.35%
Core Portfolio \| Unsecured Notes Payable \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
Core Portfolio \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 258,533	260,854
Core Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 176,714	$ 178,271
Core Portfolio \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.88%	3.88%
Maturity Date	2024-02
Core Portfolio \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	6.00%	6.00%
Maturity Date	2035-04
Core Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Long-term debt, gross	$ 81,819	$ 82,583
Core Portfolio \| Mortgages \| Variable Rate - Swapped \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.41%	3.41%
Maturity Date	2023-01
Core Portfolio \| Mortgages \| Variable Rate - Swapped \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	5.67%	5.67%
Maturity Date	2028-11
Core Portfolio \| Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross		$ 383
Core Portfolio \| Unsecured Debt \| Variable Rate Unsecured Term Loans - Swapped
Debt Instrument [Line Items]
Interest rate, stated percentage		1.25%
Maturity Date	2023-03
Long-term debt, gross	$ 350,000	$ 349,617
Core Portfolio \| Unsecured Debt \| Variable Rate Unsecured Term Loans - Swapped \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	2.49%	2.54%
Core Portfolio \| Unsecured Debt \| Variable Rate Unsecured Term Loans - Swapped \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.05%	3.59%
Core Portfolio \| Line of Credit \| Unsecured Line of Credit
Debt Instrument [Line Items]
Long-term debt, gross	$ 0
Fund Portfolio \| Mortgages \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%
Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,236,805	$ 1,001,658
Variable Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 427,337	558,675
Fund II \| Fund Portfolio
Debt Instrument [Line Items]
Maturity Date	2020-12
Long-term debt, gross	$ 23,925
Fund II \| Fund Portfolio \| Variable Rate Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.39%
Fund II \| Fund Portfolio \| Mortgages
Debt Instrument [Line Items]
Maturity Date	2022-03
Long-term debt, gross	$ 243,063	224,587
Fund II \| Fund Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Interest rate, stated percentage	4.75%
Maturity Date	2020-05
Long-term debt, gross	$ 200,000	$ 205,262
Fund II \| Fund Portfolio \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage		1.00%
Fund II \| Fund Portfolio \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage		4.75%
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Interest rate, stated percentage	4.27%	4.27%
Maturity Date	2021-11
Long-term debt, gross	$ 19,138	$ 19,325
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Fund II \| Fund Portfolio \| Unsecured Debt \| Variable Rate Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%	1.40%
Fund II \| Fund Portfolio \| Unsecured Debt \| Unsecured Notes Payable
Debt Instrument [Line Items]
Long-term debt, gross	$ 40,000	$ 40,000
Fund II \| Fund Portfolio \| Unsecured Debt \| Unsecured Notes Payable \| LIBOR
Debt Instrument [Line Items]
Maturity Date	2020-09
Fund III \| Fund Portfolio
Debt Instrument [Line Items]
Maturity Date	2020-07
Fund III \| Fund Portfolio \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Fund III \| Fund Portfolio \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.10%
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 74,145	$ 90,096
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	3.00%
Maturity Date	2020-01
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.10%	4.65%
Maturity Date	2020-06
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate		0.50%
Fund IV \| Fund Portfolio
Debt Instrument [Line Items]
Maturity Date	2021-02
Fund IV \| Fund Portfolio \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%
Fund IV \| Fund Portfolio \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.40%
Fund IV \| Fund Portfolio \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 253,990	$ 313,095
Fund IV \| Fund Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 8,189	$ 8,189
Fund IV \| Fund Portfolio \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.40%	3.40%
Maturity Date	2025-10
Fund IV \| Fund Portfolio \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%	4.50%
Maturity Date	2026-06
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Long-term debt, gross	$ 88,601	$ 71,841
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| Minimum
Debt Instrument [Line Items]
Maturity Date	2020-03
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| Maximum
Debt Instrument [Line Items]
Maturity Date	2022-12
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	3.67%	3.67%
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	4.61%	4.23%
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 157,200	$ 233,065
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
Maturity Date	2019-12
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.40%	3.95%
Maturity Date	2022-04
Fund IV \| Fund Portfolio \| Unsecured Debt \| Term Loan / Subscription Facility
Debt Instrument [Line Items]
Long-term debt, gross	$ 87,625	$ 40,825
Fund IV \| Fund Portfolio \| Unsecured Debt \| Term Loan / Subscription Facility \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%	1.65%
Maturity Date	2019-12
Fund IV \| Fund Portfolio \| Unsecured Debt \| Term Loan / Subscription Facility \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%	2.75%
Maturity Date	2021-06
Fund V \| Fund Portfolio \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	2.15%
Fund V \| Fund Portfolio \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Fund V \| Fund Portfolio \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 208,406	$ 138,076
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Long-term debt, gross	$ 156,900	$ 86,570
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| Minimum
Debt Instrument [Line Items]
Maturity Date	2021-02
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| Maximum
Debt Instrument [Line Items]
Maturity Date	2024-03
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	4.01%	4.61%
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	4.78%	4.78%
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 51,506	$ 51,506
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		2.25%
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	2.15%
Maturity Date	2020-10
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Maturity Date	2021-01
Fund V \| Fund Portfolio \| Unsecured Debt \| Subscription Line
Debt Instrument [Line Items]
Long-term debt, gross	$ 148,380	$ 102,800
Fund V \| Fund Portfolio \| Unsecured Debt \| Subscription Line \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
Maturity Date	2020-05

Debt instrument, maturity month and year.

No definition available.

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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

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.

+ References

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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, after accumulated amortization, of debt premium.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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