CAPITAL REQUIREMENTS AND RESTRICTIONS ON RETAINED EARNINGS - Reconciliation of Capital Amounts (Details) $ in Thousands	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Reconciliation of capital amounts [Abstract]
Total stockholders' equity	$ 645,140	$ 871,884	$ 847,308	$ 843,958
Adjustments:
LESS: Goodwill, net of associated deferred tax liabilities	299,186
LESS: Certain other intangible assets	26,406
LESS: Net deferred tax assets from operating loss and tax credit carry-forwards	17,968
LESS: Net unrealized gains (losses) on available for sale securities	(211,600)
LESS: Noncontrolling interest	(30)	$ 1,155
ADD: Adoption of Accounting Standards Update 2016-13	2,689
Common Equity Tier 1	515,899
Long-term borrowings and other instruments qualifying as Tier 1	13,661
Tier 1 minority interest not included in common equity Tier 1 capital	(20)
Total Tier 1 capital	529,540
Allowance for credit losses	43,623
Subordinated debentures, net of issuance costs	20,000
Total capital	$ 593,163
Common Equity Tier 1, risk-based (as a percent)	0.070
Tier 1 risk-based (as a percent)	0.085
Total risk based capital ratios with buffer (as a percent)	0.105
Capital conservation buffer requirement	0.025