Loans Receivable - Schedule of Loans Receivable (Details) - USD ($)		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 14,301,358,000			$ 13,312,388,000
Ratio of loan type to all loans		100.00%			100.00%
Allowance for credit losses on loans		$ 165,349,000	$ 157,101,000	$ 147,513,000	$ 131,534,000	$ 134,022,000	$ 133,086,000	$ 129,257,000
Loans in process		1,353,774,000			1,201,341,000
Net deferred fees, costs and discounts		48,809,000			48,938,000
Total loan contra accounts		1,567,932,000			1,381,813,000
Net loans	[1]	12,733,426,000			11,930,575,000
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 7,900,191,000			$ 6,564,698,000
Ratio of loan type to all loans		55.20%			49.30%
Allowance for credit losses on loans		$ 106,408,000	98,704,000	89,581,000	$ 93,470,000	94,803,000	93,546,000	88,178,000
Commercial loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 1,510,099,000			$ 1,422,674,000
Ratio of loan type to all loans		10.60%			10.70%
Allowance for credit losses on loans		$ 12,088,000	11,742,000	10,506,000	$ 7,391,000	7,283,000	7,394,000	8,329,000
Commercial loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 1,707,893,000			$ 1,631,170,000
Ratio of loan type to all loans		11.90%			12.30%
Allowance for credit losses on loans		$ 15,807,000	14,639,000	13,067,000	$ 13,170,000	13,004,000	12,448,000	11,852,000
Commercial loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 2,158,000,000			$ 1,268,695,000
Ratio of loan type to all loans		15.10%			9.50%
Allowance for credit losses on loans		$ 42,179,000	38,576,000	33,676,000	$ 31,450,000	31,071,000	30,574,000	28,702,000
Commercial loans | Commercial & industrial | SBA Payroll Protection Program, CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		758,955,000
Allowance for credit losses on loans		0
Commercial loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 2,328,987,000			$ 2,038,052,000
Ratio of loan type to all loans		16.30%			15.30%
Allowance for credit losses on loans		$ 25,693,000	23,348,000	21,919,000	$ 32,304,000	34,057,000	33,396,000	31,317,000
Commercial loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 195,212,000			$ 204,107,000
Ratio of loan type to all loans		1.40%			1.50%
Allowance for credit losses on loans		$ 10,641,000	10,399,000	10,413,000	$ 9,155,000	9,388,000	9,734,000	7,978,000
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 6,401,167,000			$ 6,747,690,000
Ratio of loan type to all loans		44.80%			50.70%
Allowance for credit losses on loans		$ 58,941,000	58,397,000	57,932,000	$ 38,064,000	39,219,000	39,540,000	41,079,000
Consumer loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 607,329,000			$ 540,741,000
Ratio of loan type to all loans		4.20%			4.10%
Allowance for credit losses on loans		$ 3,336,000	3,175,000	2,930,000	$ 1,369,000	1,884,000	1,976,000	1,842,000
Consumer loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 5,461,605,000			$ 5,835,194,000
Ratio of loan type to all loans		38.20%			43.80%
Allowance for credit losses on loans		$ 47,149,000	46,817,000	46,356,000	$ 30,988,000	31,514,000	31,476,000	33,033,000
Consumer loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 100,102,000			$ 99,694,000
Ratio of loan type to all loans		0.70%			0.70%
Allowance for credit losses on loans		$ 2,671,000	2,578,000	2,567,000	$ 2,143,000	2,032,000	2,076,000	2,164,000
Consumer loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 140,636,000			$ 142,178,000
Ratio of loan type to all loans		1.00%			1.10%
Allowance for credit losses on loans		$ 2,588,000	2,246,000	2,034,000	$ 1,103,000	1,087,000	1,082,000	781,000
Consumer loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 91,495,000			$ 129,883,000
Ratio of loan type to all loans		0.60%			1.00%
Allowance for credit losses on loans		$ 3,197,000	$ 3,581,000	$ 4,045,000	$ 2,461,000	$ 2,702,000	$ 2,930,000	$ 3,259,000

[1] Effective October 1, 2019, the Company has applied FASB ASU 2016-13, Financial Instruments - Credit Losses ("ASC 326"), so the allowance calculation is based on current expected credit loss methodology ("CECL"). Prior to October 1, 2019, the calculation was based on incurred loss methodology. See Note E "Allowance for Losses on Loans" for details.