New Accounting Pronouncements - ASU and Change in Accounting Principle (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans	$ 165,349	$ 157,101	$ 147,513		$ 131,534	$ 134,022	$ 133,086	$ 129,257
Reserve for unfunded commitments	19,500	18,650	18,250		6,900
Total allowance for credit losses	184,849	175,751	165,763	$ 166,934	138,434
Decrease to retained earnings	1,990,509	1,964,612			2,032,995	2,012,734	2,004,280	1,996,908
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings	1,403,124	1,384,833			1,335,909	1,300,116	1,262,236	1,188,971
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				17,750	17,750
Reserve for unfunded commitments					10,750
Total allowance for credit losses					28,500
Decrease to retained earnings					(21,945)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings, Pre-Tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings					(28,500)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings, Tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings					6,555
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings				(21,945)	(21,945)
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	106,408	98,704	89,581		93,470	94,803	93,546	88,178
Commercial loans | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				(4,135)	(4,135)
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	12,088	11,742	10,506		7,391	7,283	7,394	8,329
Commercial loans | Multi-family | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					3,013
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	15,807	14,639	13,067		13,170	13,004	12,448	11,852
Commercial loans | Commercial real estate | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					(146)
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	42,179	38,576	33,676		31,450	31,071	30,574	28,702
Commercial loans | Commercial & industrial | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					785
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	25,693	23,348	21,919		32,304	34,057	33,396	31,317
Commercial loans | Construction | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					(9,536)
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	10,641	10,399	10,413		9,155	9,388	9,734	7,978
Commercial loans | Land - acquisition & development | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					1,749
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	58,941	58,397	57,932		38,064	39,219	39,540	41,079
Consumer loans | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans				21,885	21,885
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	3,336	3,175	2,930		1,369	1,884	1,976	1,842
Consumer loans | Construction | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					1,511
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	47,149	46,817	46,356		30,988	31,514	31,476	33,033
Consumer loans | Single-family residential | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					16,783
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	2,671	2,578	2,567		2,143	2,032	2,076	2,164
Consumer loans | Land - consumer lot loans | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					492
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	2,588	2,246	2,034		1,103	1,087	1,082	781
Consumer loans | HELOC | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					945
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	$ 3,197	$ 3,581	$ 4,045		2,461	$ 2,702	$ 2,930	$ 3,259
Consumer loans | Consumer | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					$ 2,154
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