Debt - Summary of Consolidated Indebtedness (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,438,386	$ 1,505,671
Mortgage and other notes payable, net	909,174	1,055,728
Unsecured notes payable, net	473,735	432,990
Unsecured line of credit	41,500	0
Net unamortized debt issuance costs	(14,833)	(18,289)
Unamortized fair market value of assumed debt	856	1,336
Total indebtedness	1,424,409	1,488,718
Letters of credit, outstanding amount	19,700	11,300
Total Debt - Fixed and Effectively Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	899,650	860,486
Total Debt - Variable Rate
Debt Instrument [Line Items]
Long-term debt, gross	538,736	645,185
Total Debt - Variable Rate Fixed and Effectively Fixed Rate During Period
Debt Instrument [Line Items]
Long-term debt, gross	504,018	365,343
Variable-rate debt that subject to interest cap agreements
Debt Instrument [Line Items]
Long-term debt, gross	141,100	186,600
Mortgages
Debt Instrument [Line Items]
Mortgage and other notes payable, net	909,174	1,055,728
Net unamortized debt issuance costs	(12,943)	(16,642)
Unamortized fair market value of assumed debt	$ 856	1,336
Mortgages \| LIBOR
Debt Instrument [Line Items]
Interest rate, stated percentage	3.90%
Unsecured Debt
Debt Instrument [Line Items]
Unsecured notes payable, net	$ 473,735	432,990
Net unamortized debt issuance costs	(1,890)	(1,646)
Line of Credit
Debt Instrument [Line Items]
Unsecured line of credit	$ 41,500	0
Core Portfolio \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%
Core Portfolio \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.90%
Core Portfolio \| Variable Rate Unsecured Term Loans - Swapped \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.30%
Core Portfolio \| Variable Rate Unsecured Term Loans - Swapped \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%
Core Portfolio \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 254,022	317,125
Core Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 179,870	$ 234,875
Core Portfolio \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.88%	3.88%
Core Portfolio \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	5.89%	5.89%
Core Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Long-term debt, gross	$ 74,152	$ 82,250
Core Portfolio \| Mortgages \| Variable Rate - Swapped \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	1.71%	1.71%
Core Portfolio \| Mortgages \| Variable Rate - Swapped \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.77%	3.77%
Core Portfolio \| Unsecured Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 300,000	$ 300,000
Core Portfolio \| Unsecured Debt \| Unsecured Term Loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 51,194
Core Portfolio \| Unsecured Debt \| Unsecured Term Loans \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.30%	1.30%
Core Portfolio \| Unsecured Debt \| Unsecured Term Loans \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
Core Portfolio \| Unsecured Debt \| Variable Rate Unsecured Term Loans - Swapped
Debt Instrument [Line Items]
Long-term debt, gross	$ 300,000	$ 248,806
Core Portfolio \| Unsecured Debt \| Variable Rate Unsecured Term Loans - Swapped \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	1.24%	1.24%
Core Portfolio \| Unsecured Debt \| Variable Rate Unsecured Term Loans - Swapped \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.77%	3.77%
Core Portfolio \| Line of Credit \| Unsecured Line of Credit
Debt Instrument [Line Items]
Long-term debt, gross	$ 18,048	$ 0
Core Portfolio \| Line of Credit \| Unsecured Line of Credit \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%	1.40%
Core Portfolio \| Line of Credit \| Unsecured Line of Credit - Swapped
Debt Instrument [Line Items]
Long-term debt, gross	$ 23,452	$ 0
Core Portfolio \| Line of Credit \| Unsecured Line of Credit - Swapped \| LIBOR \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	1.24%
Core Portfolio \| Line of Credit \| Unsecured Line of Credit - Swapped \| LIBOR \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.77%
Fund II \| Fund Portfolio \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.39%
Fund II \| Fund Portfolio \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 224,822	412,291
Fund II \| Fund Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 205,262	$ 249,762
Fund II \| Fund Portfolio \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	1.00%	1.00%
Fund II \| Fund Portfolio \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.75%	5.80%
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Interest rate, stated percentage	2.88%	2.88%
Long-term debt, gross	$ 19,560	$ 19,779
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 0	$ 142,750
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.39%
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate		0.62%
Fund II \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate		2.50%
Fund II \| Fund Portfolio \| Unsecured Debt \| Unsecured Notes Payable
Debt Instrument [Line Items]
Long-term debt, gross	$ 31,500	$ 0
Fund II \| Fund Portfolio \| Unsecured Debt \| Unsecured Notes Payable \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%
Fund III \| Fund Portfolio \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%
Fund III \| Fund Portfolio \| Variable Rate Debt \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 65,866	$ 83,467
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%	4.65%
Fund III \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%	0.50%
Fund IV \| Fund Portfolio \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%
Fund IV \| Fund Portfolio \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%
Fund IV \| Fund Portfolio \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 347,938	$ 258,151
Fund IV \| Fund Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 10,503	$ 10,503
Fund IV \| Fund Portfolio \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.40%	3.40%
Fund IV \| Fund Portfolio \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%	4.50%
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Interest rate, stated percentage	1.78%	1.78%
Long-term debt, gross	$ 86,851	$ 14,509
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 250,584	$ 233,139
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%	1.70%
Fund IV \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%	3.95%
Fund IV \| Fund Portfolio \| Unsecured Debt \| Term Loan / Subscription Facility
Debt Instrument [Line Items]
Long-term debt, gross	$ 40,825	$ 134,636
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%
Fund IV \| Fund Portfolio \| Unsecured Debt \| Term Loan / Subscription Facility \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%	2.75%
Fund V \| Unsecured Debt \| Subscription Line \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 28,613	$ 0
Fund V \| Fund Portfolio \| Mortgages \| Variable Rate Debt \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.25%
Fund V \| Fund Portfolio \| Unsecured Debt \| Subscription Line
Debt Instrument [Line Items]
Long-term debt, gross	$ 103,300	$ 0
Fund V \| Fund Portfolio \| Unsecured Debt \| Subscription Line \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
Revolving Credit Facility \| Core Portfolio \| Unsecured Debt
Debt Instrument [Line Items]
Letters of credit, outstanding amount	$ 12,300	$ 11,300

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

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
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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.

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-Publisher FASB
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- Definition

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

+ References

No definition available.

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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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- 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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