Acquisitions and Dispositions - Property Dispositions (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2018 USD ($) property property_Unit	Jun. 30, 2017 USD ($) property
Business Acquisition [Line Items]
No. of Units | property_Unit	10,121
Property dispositions
Business Acquisition [Line Items]
No. of Units | property	1,369	1,017
Sales Price	$ 170,920	$ 130,710
Gain on Sale	64,500	35,838
Non-controlling partner portion of gain	$ 27,644	$ 17,125
Property dispositions | Melbourne, FL
Business Acquisition [Line Items]
No. of Units | property	208
Sales Price	$ 22,250
Gain on Sale	12,519
Non-controlling partner portion of gain	$ 2,504
Property dispositions | Palm Beach Gardens, FL
Business Acquisition [Line Items]
No. of Units | property	542
Sales Price	$ 97,200
Gain on Sale	41,830
Non-controlling partner portion of gain	$ 20,593
Property dispositions | Valley, AL
Business Acquisition [Line Items]
No. of Units | property	618
Sales Price	$ 51,000
Gain on Sale	9,712
Non-controlling partner portion of gain	$ 4,547
Property dispositions | New York, NY
Business Acquisition [Line Items]
No. of Units | property	1	1
Sales Price	$ 470	$ 465
Gain on Sale	439	449
Non-controlling partner portion of gain	$ 0	$ 0
Property dispositions | Greenville, SC
Business Acquisition [Line Items]
No. of Units | property		350
Sales Price		$ 68,000
Gain on Sale		18,483
Non-controlling partner portion of gain		$ 9,329
Property dispositions | Panama City, FL
Business Acquisition [Line Items]
No. of Units | property		160
Sales Price		$ 14,720
Gain on Sale		7,393
Non-controlling partner portion of gain		$ 3,478
Property dispositions | Atlanta, GA
Business Acquisition [Line Items]
No. of Units | property		350
Sales Price		$ 36,750
Gain on Sale		8,905
Non-controlling partner portion of gain		$ 4,166
Property dispositions | Hixon, TN
Business Acquisition [Line Items]
No. of Units | property		156
Sales Price		$ 10,775
Gain on Sale		608
Non-controlling partner portion of gain		$ 152