Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,527,591	$ 4,353,121
Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,490,952	4,307,239
Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,383	14,947
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	29,256	30,935
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,689,375	1,578,835
Real estate loans | One-to-four family | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,680,034	1,569,064
Real estate loans | One-to-four family | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | One-to-four family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,341	9,771
Real estate loans | One-to-four family | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,888,745	1,821,982
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,871,631	1,796,786
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,764	11,600
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,350	13,596
Real estate loans | Multifamily and commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Construction
Financing Receivable, Recorded Investment [Line Items]
Total loans	241,699	218,408
Real estate loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	241,699	218,408
Real estate loans | Construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Real estate loans | Construction | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial business loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	273,325	267,664
Commercial business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	265,153	258,454
Commercial business loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,619	3,347
Commercial business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,553	5,863
Commercial business loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment [Line Items]
Total loans	433,108	464,962
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	431,096	463,257
Consumer loans | Home equity loans and advances | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Home equity loans and advances | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,012	1,705
Consumer loans | Home equity loans and advances | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,339	1,270
Consumer loans | Other consumer loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,339	1,270
Consumer loans | Other consumer loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Other consumer loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer loans | Other consumer loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0