Allowance for Losses on Loans - ASU and Change in Accounting Principle (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				$ 134,022		$ 134,022	$ 165,349	$ 157,101	$ 147,513		$ 131,534	$ 133,086	$ 129,257
Reserve for unfunded commitments							19,500	18,650	18,250		6,900
Total allowance for credit losses	$ 175,751	$ 175,751	$ 165,763		$ 184,849		184,849	175,751	165,763	$ 166,934	138,434
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	175,751	165,763	138,434		138,434
Net (charge-offs) recoveries	(1,702)	1,788	2,579	936	2,665	3,515
Net provision (release)	10,800	8,200	(3,750)
Ending balance	$ 184,849	$ 175,751	$ 165,763		$ 184,849
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										149,284
Reserve for unfunded commitments										17,650
Total allowance for credit losses										166,934
Commercial loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				94,803		94,803	106,408	98,704	89,581		93,470	93,546	88,178
Commercial loans | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										89,335
Commercial loans | Multi-family
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				7,283		7,283	12,088	11,742	10,506		7,391	7,394	8,329
Commercial loans | Multi-family | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										10,404
Commercial loans | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				13,004		13,004	15,807	14,639	13,067		13,170	12,448	11,852
Commercial loans | Commercial real estate | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										13,024
Commercial loans | Commercial & industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				31,071		31,071	42,179	38,576	33,676		31,450	30,574	28,702
Commercial loans | Commercial & industrial | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										32,235
Commercial loans | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				34,057		34,057	25,693	23,348	21,919		32,304	33,396	31,317
Commercial loans | Construction | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										22,768
Commercial loans | Land - acquisition & development
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				9,388		9,388	10,641	10,399	10,413		9,155	9,734	7,978
Commercial loans | Land - acquisition & development | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										10,904
Consumer loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				39,219		39,219	58,941	58,397	57,932		38,064	39,540	41,079
Consumer loans | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										59,949
Consumer loans | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				1,884		1,884	3,336	3,175	2,930		1,369	1,976	1,842
Consumer loans | Construction | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										2,880
Consumer loans | Single-family residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				31,514		31,514	47,149	46,817	46,356		30,988	31,476	33,033
Consumer loans | Single-family residential | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										47,771
Consumer loans | Land - consumer lot loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				2,032		2,032	2,671	2,578	2,567		2,143	2,076	2,164
Consumer loans | Land - consumer lot loans | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										2,635
Consumer loans | HELOC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				1,087		1,087	2,588	2,246	2,034		1,103	1,082	781
Consumer loans | HELOC | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										2,048
Consumer loans | Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				$ 2,702		$ 2,702	$ 3,197	$ 3,581	$ 4,045		$ 2,461	$ 2,930	$ 3,259
Consumer loans | Consumer | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans										$ 4,615