FAIR VALUE OF FINANCIAL INSTRUMENTS - Quantitative Information (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2022 USD ($)	Sep. 30, 2021 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Foreclosed assets, net	$ 112	$ 2,077
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Range of estimated selling cost (percentage)	4.00%
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Range of estimated selling cost (percentage)	35.00%
Valuation, Market Approach | Minimum | Discount Rate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Measurement input	0.04
Valuation, Market Approach | Maximum | Discount Rate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Measurement input	0.35
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 0	0
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	3,642,987	3,616,646
Total loans and leases, net individually evaluated for credit loss | Level 3 | Valuation, Market Approach
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 8,006	12,775
Total loans and leases, net individually evaluated for credit loss | Level 3 | Valuation, Market Approach | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Range of estimated selling cost (percentage)	4.00%
Total loans and leases, net individually evaluated for credit loss | Level 3 | Valuation, Market Approach | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Range of estimated selling cost (percentage)	35.00%
Foreclosed assets, net | Level 3 | Valuation, Market Approach
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 112	2,077
Foreclosed assets, net | Level 3 | Valuation, Market Approach | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Range of estimated selling cost (percentage)	9.00%
Foreclosed assets, net | Level 3 | Valuation, Market Approach | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Range of estimated selling cost (percentage)	20.00%
Nonrecurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 8,118	14,852
Nonrecurring | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Nonrecurring | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Nonrecurring | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	8,118	14,852
Nonrecurring | Commercial finance
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	8,006	3,404
Nonrecurring | Commercial finance | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Nonrecurring | Commercial finance | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Nonrecurring | Commercial finance | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	8,006	3,404
Nonrecurring | Community banking
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value		9,371
Nonrecurring | Community banking | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value		0
Nonrecurring | Community banking | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value		0
Nonrecurring | Community banking | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value		9,371
Nonrecurring | Total loans and leases, net individually evaluated for credit loss
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	8,006	12,775
Nonrecurring | Total loans and leases, net individually evaluated for credit loss | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Nonrecurring | Total loans and leases, net individually evaluated for credit loss | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	0	0
Nonrecurring | Total loans and leases, net individually evaluated for credit loss | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	8,006	12,775
Nonrecurring | Foreclosed assets, net
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Foreclosed assets, net	112	2,077
Nonrecurring | Foreclosed assets, net | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Foreclosed assets, net	0	0
Nonrecurring | Foreclosed assets, net | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Foreclosed assets, net	0	0
Nonrecurring | Foreclosed assets, net | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Foreclosed assets, net	$ 112	$ 2,077