Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jun. 30, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 12,898,775	$ 13,312,388
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 12,319,108	$ 13,208,853
Pass | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	95.51%	99.22%
Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 298,341	$ 20,879
Special mention | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	2.31%	0.16%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 280,344	$ 82,656
Substandard | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	2.17%	0.62%
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 982	$ 0
Doubtful | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.01%	0.00%
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 0	$ 0
Loss | Accounts Receivable | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.00%	0.00%
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 6,836,049	$ 6,564,698
Commercial loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	6,285,205	6,489,697
Commercial loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	298,341	20,879
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	251,521	54,122
Commercial loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	982	0
Commercial loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,509,837	1,422,674
Commercial loans | Multi-family | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,478,551	1,418,837
Commercial loans | Multi-family | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	13,749	0
Commercial loans | Multi-family | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	17,537	3,837
Commercial loans | Multi-family | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Multi-family | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,700,568	1,631,170
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,513,394	1,602,634
Commercial loans | Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	117,753	2,754
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	69,421	25,782
Commercial loans | Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,138,462	1,268,695
Commercial loans | Commercial & industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,941,389	1,229,891
Commercial loans | Commercial & industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	59,747	18,125
Commercial loans | Commercial & industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	136,344	20,679
Commercial loans | Commercial & industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	982	0
Commercial loans | Commercial & industrial | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,334,948	2,038,052
Commercial loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,217,628	2,038,052
Commercial loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	91,519	0
Commercial loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	25,801	0
Commercial loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	152,234	204,107
Commercial loans | Land - acquisition & development | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	134,243	200,283
Commercial loans | Land - acquisition & development | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	15,573	0
Commercial loans | Land - acquisition & development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,418	3,824
Commercial loans | Land - acquisition & development | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans | Land - acquisition & development | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	6,062,726	6,747,690
Consumer loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	6,033,903	6,719,156
Consumer loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	28,823	28,534
Consumer loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	278,182	540,741
Consumer loans | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	278,182	540,741
Consumer loans | Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Construction | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	5,452,424	5,835,194
Consumer loans | Single-family residential | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	5,426,365	5,808,444
Consumer loans | Single-family residential | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Single-family residential | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	26,059	26,750
Consumer loans | Single-family residential | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Single-family residential | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	99,255	99,694
Consumer loans | Land - consumer lot loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	98,976	98,828
Consumer loans | Land - consumer lot loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Land - consumer lot loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	279	866
Consumer loans | Land - consumer lot loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Land - consumer lot loans | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	141,165	142,178
Consumer loans | HELOC | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	138,695	141,271
Consumer loans | HELOC | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | HELOC | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,470	907
Consumer loans | HELOC | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | HELOC | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	91,700	129,883
Consumer loans | Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	91,685	129,872
Consumer loans | Consumer | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	15	11
Consumer loans | Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans | Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 0	$ 0