Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Allowance for loan losses:
Beginning balance	$ 54,633	$ 51,867
Provision for loan losses	3,400	0
Recoveries	188	28
Charge-offs	(43)	(46)
Ending balance	58,178	51,849
Real estate loans \| One to four family
Allowance for loan losses:
Beginning balance	18,533	18,638
Provision for loan losses	1,473	(27)
Recoveries	9	3
Charge-offs	(24)	(15)
Ending balance	19,991	18,599
Real estate loans \| Multifamily and commercial
Allowance for loan losses:
Beginning balance	18,029	17,390
Provision for loan losses	1,906	226
Recoveries	0	0
Charge-offs	(2)	0
Ending balance	19,933	17,616
Real estate loans \| Construction
Allowance for loan losses:
Beginning balance	5,299	5,960
Provision for loan losses	(82)	(1,362)
Recoveries	0	0
Charge-offs	0	0
Ending balance	5,217	4,598
Commercial business loans
Allowance for loan losses:
Beginning balance	8,480	5,721
Provision for loan losses	(373)	641
Recoveries	171	19
Charge-offs	(3)	(23)
Ending balance	8,275	6,358
Consumer loans \| Home equity loans and advances
Allowance for loan losses:
Beginning balance	4,190	4,052
Provision for loan losses	389	177
Recoveries	6	6
Charge-offs	(9)	(4)
Ending balance	4,576	4,231
Consumer loans \| Other consumer loans
Allowance for loan losses:
Beginning balance	8	11
Provision for loan losses	3	4
Recoveries	2	0
Charge-offs	(5)	(4)
Ending balance	8	11
Unallocated
Allowance for loan losses:
Beginning balance	94	95
Provision for loan losses	84	341
Recoveries	0	0
Charge-offs	0	0
Ending balance	$ 178	$ 436

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

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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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