Loans Receivable and Allowance for Loan Losses - Aging of Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 12,617	$ 13,387
Current	4,514,974	4,339,734
Total	4,527,591	4,353,121
30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6,857	5,362
60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,034	1,630
Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,726	6,395
Real estate loans | One-to-four family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	9,472	8,352
Current	1,679,903	1,570,483
Total	1,689,375	1,578,835
Real estate loans | One-to-four family | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5,201	3,924
Real estate loans | One-to-four family | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,706	932
Real estate loans | One-to-four family | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,565	3,496
Real estate loans | Multifamily and commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	622	1,633
Current	1,888,123	1,820,349
Total	1,888,745	1,821,982
Real estate loans | Multifamily and commercial | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	504	0
Real estate loans | Multifamily and commercial | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	123
Real estate loans | Multifamily and commercial | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	118	1,510
Real estate loans | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Current	241,699	218,408
Total	241,699	218,408
Real estate loans | Construction | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Real estate loans | Construction | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Real estate loans | Construction | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Commercial business loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	762	1,426
Current	272,563	266,238
Total	273,325	267,664
Commercial business loans | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	355	0
Commercial business loans | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	388
Commercial business loans | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	407	1,038
Consumer loans | Home equity loans and advances
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,754	1,975
Current	431,354	462,987
Total	433,108	464,962
Consumer loans | Home equity loans and advances | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	797	1,437
Consumer loans | Home equity loans and advances | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	321	187
Consumer loans | Home equity loans and advances | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	636	351
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	7	1
Current	1,332	1,269
Total	1,339	1,270
Consumer loans | Other consumer loans | 30-59 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	1
Consumer loans | Other consumer loans | 60-89 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	7	0
Consumer loans | Other consumer loans | Greater than 90 days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	$ 0