Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Allowance for loan losses:
Beginning balance	$ 58,178	$ 51,849	$ 54,633	$ 51,867
Provision for loan losses	2,000	375	5,400	375
Recoveries	181	106	371	135
Charge-offs	(407)	(422)	(452)	(469)
Ending balance	59,952	51,908	59,952	51,908
Real estate loans \| One-to-four family
Allowance for loan losses:
Beginning balance	19,991	18,599	18,533	18,638
Provision for loan losses	(1,229)	380	245	353
Recoveries	120	22	130	26
Charge-offs	(54)	(183)	(80)	(199)
Ending balance	18,828	18,818	18,828	18,818
Real estate loans \| Multifamily and commercial
Allowance for loan losses:
Beginning balance	19,933	17,616	18,029	17,390
Provision for loan losses	(707)	189	1,199	415
Recoveries	0	0	0	0
Charge-offs	(129)	(167)	(131)	(167)
Ending balance	19,097	17,638	19,097	17,638
Real estate loans \| Construction
Allowance for loan losses:
Beginning balance	5,217	4,598	5,299	5,960
Provision for loan losses	1,354	(345)	1,272	(1,707)
Recoveries	3	0	3	0
Charge-offs	0	0	0	0
Ending balance	6,574	4,253	6,574	4,253
Commercial business loans
Allowance for loan losses:
Beginning balance	8,275	6,358	8,480	4,052
Provision for loan losses	2,697	579	2,322	2,889
Recoveries	52	77	225	96
Charge-offs	(224)	0	(227)	(23)
Ending balance	10,800	7,014	10,800	7,014
Consumer loans \| Home equity loans and advances
Allowance for loan losses:
Beginning balance	4,576	4,231	4,190	5,721
Provision for loan losses	(657)	(39)	(267)	(1,531)
Recoveries	5	7	10	13
Charge-offs	0	(70)	(9)	(74)
Ending balance	3,924	4,129	3,924	4,129
Consumer loans \| Other consumer loans
Allowance for loan losses:
Beginning balance	8	11	8	11
Provision for loan losses	(2)	1	1	5
Recoveries	1	0	3	0
Charge-offs	0	(2)	(5)	(6)
Ending balance	7	10	7	10
Unallocated
Allowance for loan losses:
Beginning balance	178	436	94	95
Provision for loan losses	544	(390)	628	(49)
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Ending balance	$ 722	$ 46	$ 722	$ 46

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

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-Name Accounting Standards Codification
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Amount of expense related to estimated loss from loan and lease transactions.

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