Organization and Background (Details)	9 Months Ended
Jun. 30, 2016 USD ($) state property property_unit	Sep. 30, 2015 USD ($)
Organization, background and significant accounting policies
Number of properties \| property	31
Number of units \| property_unit	8,973
Number of states \| state	11
Number of units under construction \| property	271
Number of units in lease up stage \| property	350
Real Estate Investment Property, Net	$ 668,985,000	$ 591,727,000
Real Estate Investment Property, Net	29,968,000
Mortgage Loans on Real Estate	19,500,000
Multi-family
Organization, background and significant accounting policies
Real Estate Investment Property, Net	$ 685,537,000
Corporate Joint Venture \| Property Acquisition \| Multi-family \| Minimum
Organization, background and significant accounting policies
Equity contribution in each transaction (as a percent)	65.00%
Corporate Joint Venture \| Property Acquisition \| Multi-family \| Maximum
Organization, background and significant accounting policies
Equity contribution in each transaction (as a percent)	80.00%

Number of Units in Real Estate Property, In Lease Up Stage

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Number of Units in Real Estate Property, Under Construction

+ References

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Total carrying amount of mortgage loans as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29.6)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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The number of real estate properties owned as of the balance sheet date.

+ References

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The number of states the entity operates in as of the balance sheet date.

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The number of units in a real estate property owned as of the balance sheet date.

+ References

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Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(d))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Amount of real estate owned for income production and capital accretion potential, not otherwise specified in the existing taxonomy. Such real estate excludes that which is occupied or used in the business, for entertainment purposes, or held in inventory for specific purposes.

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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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Percentage of subsidiary's or equity investee's stock owned by parent company after stock transaction.

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