Debt - Summary of Consolidated Indebtedness (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,519,378,000	$ 1,505,671,000
Mortgage and other notes payable, net	1,143,049,000	1,055,728,000
Unsecured notes payable, net	358,847,000	432,990,000
Net unamortized debt issuance costs	(18,640,000)	(18,289,000)
Unamortized fair market value of assumed debt	1,158,000	1,336,000
Total indebtedness	1,501,896,000	1,488,718,000
Total Debt - Fixed and Effectively Fixed Rate [Member]
Debt Instrument [Line Items]
Long-term debt, gross	926,314,000	860,486,000
Total Debt - Variable Rate [Member]
Debt Instrument [Line Items]
Long-term debt, gross	593,064,000	645,185,000
Total Debt - Variable Rate Fixed and Effectively Fixed Rate During Period [Member]
Debt Instrument [Line Items]
Long-term debt, gross	431,774	365,343
Mortgages [Member]
Debt Instrument [Line Items]
Mortgage and other notes payable, net	1,143,049,000	1,055,728,000
Net unamortized debt issuance costs	(16,951,000)	(16,642,000)
Unamortized fair market value of assumed debt	1,158,000	1,336,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Unsecured notes payable, net	358,847,000	432,990,000
Net unamortized debt issuance costs	(1,689,000)	(1,646,000)
Total indebtedness	0	9,900,000
Line of Credit [Member]
Debt Instrument [Line Items]
Unsecured line of credit	0	0
Core Portfolio [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt, gross	315,936,000	317,125,000
Core Portfolio [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Long-term debt, gross	234,273,000	234,875,000
Core Portfolio [Member] \| Mortgages [Member] \| Variable Rate - Swapped [Member]
Debt Instrument [Line Items]
Long-term debt, gross	81,663,000	82,250,000
Core Portfolio [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, gross	300,000,000	300,000,000
Core Portfolio [Member] \| Unsecured Debt [Member] \| Unsecured Term Loans [Member]
Debt Instrument [Line Items]
Long-term debt, gross	51,283,000	51,194,000
Core Portfolio [Member] \| Unsecured Debt [Member] \| Variable Rate Unsecured Term Loans - Swapped [Member]
Debt Instrument [Line Items]
Long-term debt, gross	248,717,000	248,806,000
Core Portfolio [Member] \| Line of Credit [Member] \| Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Long-term debt, gross	0	0
Fund II [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt, gross	412,238,000	412,291,000
Fund II [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 249,762,000	$ 249,762,000
Fund II [Member] \| Mortgages [Member] \| Variable Rate - Swapped [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	2.88%	2.88%
Long-term debt, gross	$ 19,726,000	$ 19,779,000
Fund II [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Long-term debt, gross	142,750,000	142,750,000
Fund III [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Long-term debt, gross	79,680,000	83,467,000
Fund IV [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt, gross	350,988,000	258,151,000
Fund IV [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 10,504,000	$ 10,503,000
Fund IV [Member] \| Mortgages [Member] \| Variable Rate - Swapped [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	1.78%	1.78%
Long-term debt, gross	$ 81,668,000	$ 14,509,000
Fund IV [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Long-term debt, gross	258,816,000	233,139,000
Fund IV [Member] \| Unsecured Debt [Member] \| Term Loan / Subscription Facility [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 60,536,000	$ 134,636,000
London Interbank Offered Rate (LIBOR) [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	2.15%
London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Line of Credit [Member] \| Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.40%	1.40%
Minimum [Member] \| Core Portfolio [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.88%	3.88%
Minimum [Member] \| Core Portfolio [Member] \| Mortgages [Member] \| Variable Rate - Swapped [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	1.71%	1.71%
Minimum [Member] \| Core Portfolio [Member] \| Unsecured Debt [Member] \| Variable Rate Unsecured Term Loans - Swapped [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	1.24%	1.24%
Minimum [Member] \| Fund II [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	1.00%	1.00%
Minimum [Member] \| Fund IV [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.40%	3.40%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.08%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Variable Rate Unsecured Term Loans - Swapped [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.30%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Unsecured Debt [Member] \| Unsecured Term Loans [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.30%	1.30%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund II [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.79%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund II [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.79%	0.62%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund IV [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund IV [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.70%	1.70%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund IV [Member] \| Unsecured Debt [Member] \| Term Loan / Subscription Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.65%	1.65%
Minimum [Member] \| Prime Rate [Member] \| Fund III [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Minimum [Member] \| Prime Rate [Member] \| Fund III [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%	0.50%
Maximum [Member] \| Core Portfolio [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	6.65%	6.65%
Maximum [Member] \| Core Portfolio [Member] \| Mortgages [Member] \| Variable Rate - Swapped [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.77%	3.77%
Maximum [Member] \| Core Portfolio [Member] \| Unsecured Debt [Member] \| Variable Rate Unsecured Term Loans - Swapped [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	3.77%	3.77%
Maximum [Member] \| Fund II [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	5.80%	5.80%
Maximum [Member] \| Fund IV [Member] \| Mortgages [Member] \| Fixed Rate [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	4.50%	4.50%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.90%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Variable Rate Unsecured Term Loans - Swapped [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Core Portfolio [Member] \| Unsecured Debt [Member] \| Unsecured Term Loans [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%	1.60%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund II [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund II [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	2.50%	2.50%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund III [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund III [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	4.65%	4.65%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund IV [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund IV [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	3.95%	3.95%
Maximum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Fund IV [Member] \| Unsecured Debt [Member] \| Term Loan / Subscription Facility [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%	2.75%

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

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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
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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.

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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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- 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
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