LOANS & ALLOWANCE FOR CREDIT LOSSES - LOAN PORTFOLIO COMPOSITION (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jul. 01, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	$ 11,055,178		$ 10,703,442
Allowance for credit losses - loans	(132,915)	$ (75,807)	(75,807)	$ (59,227)	$ (57,085)
Unaccreted premiums (discounts) and loan and lease fees	3,187		3,714
Total net loans and leases	10,925,450		10,631,349
Single Family - Mortgage & Warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	4,935,351		4,722,304
Allowance for credit losses - loans	(28,307)	(25,901)	(25,901)	(21,712)	(22,290)
Multifamily and Commercial Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	2,299,332		2,263,054
Allowance for credit losses - loans	(12,419)	(4,718)	(4,718)	(4,005)	(3,807)
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	2,443,646		2,297,920
Allowance for credit losses - loans	(49,198)	(21,052)	(21,052)	(12,462)	(14,632)
Commercial & Industrial - Non-RE
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	866,508		885,320
Allowance for credit losses - loans	(23,295)	(9,954)	(9,954)	(14,402)	(9,544)
Auto & Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	330,093		341,365
Allowance for credit losses - loans	(8,678)	(9,461)	(9,461)	(6,585)	(6,339)
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross loans and leases	180,248		193,479
Allowance for credit losses - loans	$ (11,018)	$ (4,721)	$ (4,721)	$ (61)	$ (473)