Loans Receivable and Allowance for Loan Losses - Rollforward of Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Allowance for loan losses:
Beginning balance	$ 54,633	$ 58,178	$ 51,867	$ 56,948
Provision for loan losses	3,400	6,677	6,426	417
Recoveries	188	707	584	721
Charge-offs	(43)	(3,220)	(4,244)	(6,219)
Ending balance	58,178	62,342	54,633	51,867
Real estate loans \| One-to-four family
Allowance for loan losses:
Beginning balance	18,533	19,991	18,638	16,442
Provision for loan losses	1,473	(4,503)	1,029	5,534
Recoveries	9	334	268	158
Charge-offs	(24)	(590)	(1,402)	(3,496)
Ending balance	19,991	15,232	18,533	18,638
Real estate loans \| Multifamily and commercial
Allowance for loan losses:
Beginning balance	18,029	19,933	17,390	20,352
Provision for loan losses	1,906	3,445	1,644	(2,106)
Recoveries	0	2	75	23
Charge-offs	(2)	(129)	(1,080)	(879)
Ending balance	19,933	23,251	18,029	17,390
Real estate loans \| Construction
Allowance for loan losses:
Beginning balance	5,299	5,217	5,960	6,248
Provision for loan losses	(82)	1,997	(661)	(43)
Recoveries	0	3	0	76
Charge-offs	0	0	0	(321)
Ending balance	5,217	7,217	5,299	5,960
Commercial business loans
Allowance for loan losses:
Beginning balance	8,480	8,275	5,721	7,094
Provision for loan losses	(373)	7,860	3,183	(1,323)
Recoveries	171	240	182	408
Charge-offs	(3)	(2,199)	(606)	(458)
Ending balance	8,275	14,176	8,480	5,721
Consumer loans \| Home equity loans and advances
Allowance for loan losses:
Beginning balance	4,190	4,576	4,052	6,111
Provision for loan losses	389	(1,949)	1,219	(1,061)
Recoveries	6	122	59	55
Charge-offs	(9)	(291)	(1,140)	(1,053)
Ending balance	4,576	2,458	4,190	4,052
Consumer loans \| Other consumer loans
Allowance for loan losses:
Beginning balance	8	8	11	4
Provision for loan losses	3	5	13	18
Recoveries	2	6	0	1
Charge-offs	(5)	(11)	(16)	(12)
Ending balance	8	8	8	11
Unallocated
Allowance for loan losses:
Beginning balance	94	178	95	697
Provision for loan losses	84	(178)	(1)	(602)
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Ending balance	$ 178	$ 0	$ 94	$ 95

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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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.

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Amount of expense related to estimated loss from loan and lease transactions.

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