FAIR VALUE OF FINANCIAL INSTRUMENTS - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Financial assets [Abstract]
Debt securities available for sale	$ 1,214,723	$ 1,272,493
Debt securities held to maturity	121,899	133,470
Level 1
Financial assets [Abstract]
Cash and cash equivalents	152,189	126,545
Debt securities available for sale	0	0
Debt securities held to maturity	0	0
Common equities and mutual funds	2,947	2,606
Non-marketable equity securities	0	0
Loans held for sale	0	0
Loans and leases receivable	0	0
Federal Home Loan Bank stock	0	0
Accrued interest receivable	18,687	20,400
Financial liabilities [Abstract]
Deposits held for sale	0
Deposits	3,290,280	2,920,516
Overnight federal funds purchased	194,000
Overnight federal funds purchased		642,000
Federal Home Loan Bank advances	0	0
Other short- and long-term borrowings	0	0
Accrued interest payable	6,620	9,414
Level 2
Financial assets [Abstract]
Cash and cash equivalents	0	0
Debt securities available for sale	1,214,723	1,272,493
Debt securities held to maturity	121,899	133,470
Common equities and mutual funds	0	0
Non-marketable equity securities	9,500	6,500
Loans held for sale	264,266	148,777
Loans and leases receivable	0	0
Federal Home Loan Bank stock	13,796	30,916
Accrued interest receivable	0	0
Financial liabilities [Abstract]
Deposits held for sale	288,975
Deposits	1,237,096	1,417,994
Overnight federal funds purchased	0
Overnight federal funds purchased		0
Federal Home Loan Bank advances	110,733	110,691
Other short- and long-term borrowings	106,716	113,876
Accrued interest payable	0	0
Level 3
Financial assets [Abstract]
Cash and cash equivalents	0	0
Debt securities available for sale	0	0
Debt securities held to maturity	0	0
Common equities and mutual funds	0	0
Non-marketable equity securities	0	0
Loans held for sale	0	0
Loans and leases receivable	3,557,579	3,622,597
Federal Home Loan Bank stock	0	0
Accrued interest receivable	0	0
Financial liabilities [Abstract]
Deposits held for sale	0
Deposits	0	0
Overnight federal funds purchased	0
Overnight federal funds purchased		0
Federal Home Loan Bank advances	0	0
Other short- and long-term borrowings	0	0
Accrued interest payable	0	0
Carrying Amount
Financial assets [Abstract]
Cash and cash equivalents	152,189	126,545
Debt securities available for sale	1,214,723	1,272,493
Debt securities held to maturity	123,117	134,764
Common equities and mutual funds	2,947	2,606
Non-marketable equity securities	12,140	8,169
Loans held for sale	264,266	148,777
Loans and leases receivable	3,583,297	3,651,413
Federal Home Loan Bank stock	13,796	30,916
Accrued interest receivable	18,687	20,400
Financial liabilities [Abstract]
Deposits held for sale	288,975
Deposits	4,517,605	4,337,005
Overnight federal funds purchased	194,000
Overnight federal funds purchased		642,000
Federal Home Loan Bank advances	110,000	110,000
Other short- and long-term borrowings	103,070	109,857
Accrued interest payable	6,620	9,414
Estimated Fair Value
Financial assets [Abstract]
Cash and cash equivalents	152,189	126,545
Debt securities available for sale	1,214,723	1,272,493
Debt securities held to maturity	121,899	133,470
Common equities and mutual funds	2,947	2,606
Non-marketable equity securities	12,140	8,169
Loans held for sale	264,266	148,777
Loans and leases receivable	3,557,579	3,622,597
Federal Home Loan Bank stock	13,796	30,916
Accrued interest receivable	18,687	20,400
Financial liabilities [Abstract]
Deposits held for sale	288,975
Deposits	4,527,377	4,338,510
Overnight federal funds purchased	194,000
Overnight federal funds purchased		642,000
Federal Home Loan Bank advances	110,733	110,691
Other short- and long-term borrowings	106,716	113,876
Accrued interest payable	$ 6,620	$ 9,414