Debt - Summary of Consolidated Indebtedness (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,619,171	$ 1,505,670
Mortgage and other notes payable, net	1,045,877	1,055,728
Unsecured notes payable, net	497,970	432,990
Unsecured line of credit	59,000	0
Net unamortized debt issuance costs	(17,205)	(18,288)
Unamortized fair market value of assumed debt	881	1,336
Total indebtedness	1,602,847	1,488,718
Total Debt - Fixed and Effectively Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	869,829	860,484
Total Debt - Variable Rate
Debt Instrument [Line Items]
Long-term debt, gross	749,342	645,186
Total Debt - Variable Rate Fixed and Effectively Fixed Rate During Period
Debt Instrument [Line Items]
Long-term debt, gross	454,805	365,343
Mortgages
Debt Instrument [Line Items]
Mortgage and other notes payable, net	1,045,877	1,055,728
Net unamortized debt issuance costs	(15,555)	(16,642)
Unamortized fair market value of assumed debt	$ 881	1,336
Mortgages \| LIBOR
Debt Instrument [Line Items]
Interest rate, stated percentage	4.76%
Unsecured Debt
Debt Instrument [Line Items]
Unsecured notes payable, net	$ 497,970	432,990
Net unamortized debt issuance costs	(1,650)	(1,646)
Line of Credit
Debt Instrument [Line Items]
Unsecured line of credit	$ 59,000	0
Core Portfolio \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.08%
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	$ 260,751	317,125
Core Portfolio \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 180,259	$ 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	$ 80,492	$ 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	$ 26,459	$ 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	$ 273,541	$ 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	$ 59,000	$ 0
Core Portfolio \| Line of Credit \| Unsecured Line of Credit \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%	1.40%
Fund II \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.79%
Fund II \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Fund II \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 369,955	$ 412,291
Fund II \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 224,262	$ 249,762
Fund II \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	1.00%	1.00%
Fund II \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.75%	5.80%
Fund II \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Interest rate, stated percentage	2.88%	2.88%
Long-term debt, gross	$ 19,616	$ 19,779
Fund II \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 126,077	$ 142,750
Fund II \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.79%	0.62%
Fund II \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%	2.50%
Fund II \| Unsecured Debt \| Unsecured Notes Payable
Debt Instrument [Line Items]
Long-term debt, gross	$ 31,500	$ 0
Fund II \| Unsecured Debt \| Unsecured Notes Payable \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%
Fund III \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%
Fund III \| Variable Rate Debt \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Fund III \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 69,632	$ 83,467
Fund III \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%	4.65%
Fund III \| Mortgages \| Variable Rate Debt \| Prime Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%	0.50%
Fund IV \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%
Fund IV \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%
Fund IV \| Mortgages
Debt Instrument [Line Items]
Long-term debt, gross	$ 360,213	$ 258,151
Fund IV \| Mortgages \| Fixed Rate
Debt Instrument [Line Items]
Long-term debt, gross	$ 10,503	$ 10,503
Fund IV \| Mortgages \| Fixed Rate \| Minimum
Debt Instrument [Line Items]
Interest rate, stated percentage	3.40%	3.40%
Fund IV \| Mortgages \| Fixed Rate \| Maximum
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%	4.50%
Fund IV \| Mortgages \| Variable Rate - Swapped
Debt Instrument [Line Items]
Interest rate, stated percentage	1.78%	1.78%
Long-term debt, gross	$ 81,156	$ 14,509
Fund IV \| Mortgages \| Variable Rate Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 268,554	$ 233,139
Fund IV \| Mortgages \| Variable Rate Debt \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%	1.70%
Fund IV \| Mortgages \| Variable Rate Debt \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%	3.95%
Fund IV \| Unsecured Debt \| Term Loan / Subscription Facility
Debt Instrument [Line Items]
Long-term debt, gross	$ 54,920	$ 134,636
Fund IV \| Unsecured Debt \| Term Loan / Subscription Facility \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%	1.65%
Fund IV \| 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
Debt Instrument [Line Items]
Long-term debt, gross	$ 113,200	$ 0
Fund V \| Unsecured Debt \| Subscription Line \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%

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
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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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
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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 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.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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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
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