FAIR VALUE OF FINANCIAL INSTRUMENTS - Quantitative Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Sep. 30, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Foreclosed assets, net	$ 1,328	$ 29,494
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Range of estimated selling cost	4.00%
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Range of estimated selling cost	30.00%
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	$ 0	0
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	3,557,579	3,622,597
Impaired Loans | Level 3 | Valuation, Market Approach
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017	8,707
Foreclosed Assets | Level 3 | Valuation, Market Approach
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	1,328	29,494
Nonrecurring
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	14,345	38,201
Nonrecurring | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Nonrecurring | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Nonrecurring | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	14,345	38,201
Nonrecurring | Commercial Portfolio Segment
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017	8,707
Nonrecurring | Commercial Portfolio Segment | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Nonrecurring | Commercial Portfolio Segment | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Nonrecurring | Commercial Portfolio Segment | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017	8,707
Nonrecurring | Total National Lending
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017	8,707
Nonrecurring | Total National Lending | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Nonrecurring | Total National Lending | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0	0
Nonrecurring | Total National Lending | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017	8,707
Nonrecurring | Agricultural Operating
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value		8,707
Nonrecurring | Agricultural Operating | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value		0
Nonrecurring | Agricultural Operating | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value		0
Nonrecurring | Agricultural Operating | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value		8,707
Nonrecurring | Impaired Loans
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017
Nonrecurring | Impaired Loans | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0
Nonrecurring | Impaired Loans | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	0
Nonrecurring | Impaired Loans | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value	13,017
Nonrecurring | Foreclosed Assets
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Foreclosed assets, net	1,328	29,494
Nonrecurring | Foreclosed Assets | Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Foreclosed assets, net	0	0
Nonrecurring | Foreclosed Assets | Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Foreclosed assets, net	0	0
Nonrecurring | Foreclosed Assets | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Foreclosed assets, net	$ 1,328	$ 29,494