Loans Receivable and Allowance for Loan Losses - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 4,448,109	$ 4,353,121
Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,407,813	4,307,239
Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,460	14,947
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	31,836	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	1,616,259	1,578,835
Real estate loans | One to four family | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,606,672	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	9,587	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	1,871,210	1,821,982
Real estate loans | Multifamily and commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,851,772	1,796,786
Real estate loans | Multifamily and commercial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,782	11,600
Real estate loans | Multifamily and commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	14,656	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	233,652	218,408
Real estate loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	233,652	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
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment [Line Items]
Total loans	448,020	464,962
Consumer loans | Home equity loans and advances | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	446,364	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	1,656	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	998	1,270
Consumer loans | Other consumer loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	998	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
Commercial business loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	277,970	267,664
Commercial business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	268,355	258,454
Commercial business loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,678	3,347
Commercial business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,937	5,863
Commercial business loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0