Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 12,737,292	$ 12,201,387
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 12,611,439	$ 12,066,952
Total grade as a % of total gross loans	99.00%	98.90%
Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 14,537	$ 26,760
Total grade as a % of total gross loans	0.10%	0.20%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 111,316	$ 107,675
Total grade as a % of total gross loans	0.90%	0.90%
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total gross loans	0.00%	0.00%
Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total gross loans	0.00%	0.00%
Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 5,745,598	$ 5,711,004
Single-family residential | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	5,713,075	5,671,229
Single-family residential | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Single-family residential | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	32,523	39,775
Single-family residential | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Single-family residential | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,885,034	1,597,996
Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,880,415	1,594,926
Construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	55	0
Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	4,564	3,070
Construction | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	612,688	602,631
Construction - custom | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	612,688	602,540
Construction - custom | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction - custom | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	91
Construction - custom | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction - custom | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	150,936	124,308
Land - acquisition & development | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	147,864	123,028
Land - acquisition & development | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	207
Land - acquisition & development | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,072	1,073
Land - acquisition & development | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - acquisition & development | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	103,118	104,405
Land - consumer lot loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	102,476	103,787
Land - consumer lot loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	642	618
Land - consumer lot loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Land - consumer lot loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,346,534	1,303,148
Multi-family | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,343,950	1,295,261
Multi-family | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	5,795
Multi-family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	2,584	2,092
Multi-family | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Multi-family | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,435,418	1,434,610
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,402,581	1,391,996
Commercial real estate | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	4,900	5,944
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	27,937	36,670
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,133,075	1,093,360
Commercial & industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,084,175	1,054,972
Commercial & industrial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	9,582	14,814
Commercial & industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	39,318	23,574
Commercial & industrial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial & industrial | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
HELOC
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	136,766	144,850
HELOC | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	136,122	144,229
HELOC | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
HELOC | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	644	621
HELOC | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
HELOC | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	188,125	85,075
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	188,093	84,984
Consumer | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	32	91
Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Consumer | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0