Loans Receivable - Loan Modifications (Details) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2017 USD ($) contract	Mar. 31, 2016 USD ($) contract	Mar. 31, 2017 USD ($) contract	Mar. 31, 2016 USD ($) contract
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) \| contract	7	8	15	10
Recorded investment \| $	$ 1,192	$ 1,169	$ 3,452	$ 1,393
Single-family residential
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) \| contract	7	6	13	8
Recorded investment \| $	$ 1,192	$ 871	$ 3,185	$ 1,095
Land - consumer lot loans
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) \| contract	0	1	0	1
Recorded investment \| $	$ 0	$ 146	$ 0	$ 146
Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) \| contract	0	1	2	1
Recorded investment \| $	$ 0	$ 152	$ 267	$ 152