Loans Receivable and Allowance for Loan Losses - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	$ 546	$ 540
Allowance for loan losses, Collectively evaluated for impairment	57,632	54,093
Total	58,178	54,633	$ 51,849	$ 51,867
Total loans:
Total loans, Individually evaluated for impairment	22,191	25,915
Total loans, Collectively evaluated for impairment	4,425,918	4,327,206
Total	4,448,109	4,353,121
Real estate loans \| One to four family
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	423	407
Allowance for loan losses, Collectively evaluated for impairment	19,568	18,126
Total	19,991	18,533	18,599	18,638
Total loans:
Total loans, Individually evaluated for impairment	11,644	12,247
Total loans, Collectively evaluated for impairment	1,604,615	1,566,588
Total	1,616,259	1,578,835
Real estate loans \| Multifamily and commercial
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	28	35
Allowance for loan losses, Collectively evaluated for impairment	19,905	17,994
Total	19,933	18,029	17,616	17,390
Total loans:
Total loans, Individually evaluated for impairment	3,693	6,343
Total loans, Collectively evaluated for impairment	1,867,517	1,815,639
Total	1,871,210	1,821,982
Real estate loans \| Construction
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	5,217	5,299
Total	5,217	5,299	4,598	5,960
Total loans:
Total loans, Individually evaluated for impairment	0	0
Total loans, Collectively evaluated for impairment	233,652	218,408
Total	233,652	218,408
Commercial business loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	80	84
Allowance for loan losses, Collectively evaluated for impairment	8,195	8,396
Total	8,275	8,480	6,358	5,721
Total loans:
Total loans, Individually evaluated for impairment	4,263	4,327
Total loans, Collectively evaluated for impairment	273,707	263,337
Total	277,970	267,664
Consumer loans \| Home equity loans and advances
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	15	14
Allowance for loan losses, Collectively evaluated for impairment	4,561	4,176
Total	4,576	4,190	4,231	4,052
Total loans:
Total loans, Individually evaluated for impairment	2,591	2,998
Total loans, Collectively evaluated for impairment	445,429	461,964
Total	448,020	464,962
Consumer loans \| Other consumer loans
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	8	8
Total	8	8	11	11
Total loans:
Total loans, Individually evaluated for impairment	0	0
Total loans, Collectively evaluated for impairment	998	1,270
Total	998	1,270
Unallocated
Allowance for loan losses:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	178	94
Total	178	94	$ 436	$ 95
Total loans:
Total loans, Individually evaluated for impairment	0	0
Total loans, Collectively evaluated for impairment	0	0
Total	$ 0	$ 0

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Total Loans [Abstract]

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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The balance of financing receivables that were collectively evaluated for impairment.

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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, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

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