Loans - Schedule of the Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	$ 8,884,834	$ 8,874,149
Loans covered by a FDIC loss sharing agreement	29,691	31,891
Total	8,914,525	8,906,040
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	7,935,831	8,066,457
Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	365,120	375,787
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	582,276	425,685
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	1,607	6,220
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	4,716,122	4,573,084
Loans covered by a FDIC loss sharing agreement	0	0
Total	4,716,122	4,573,084
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	4,478,193	4,433,530
Commercial real estate | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	104,941	85,256
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	132,937	54,242
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	51	56
Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Agriculture
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	1,734,388	1,854,539
Loans covered by a FDIC loss sharing agreement	0	0
Total	1,734,388	1,854,539
Agriculture | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	1,157,671	1,346,436
Agriculture | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	229,606	179,965
Agriculture | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	345,674	322,327
Agriculture | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	1,437	5,811
Agriculture | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Commercial non-real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	1,553,513	1,570,215
Loans covered by a FDIC loss sharing agreement	0	0
Total	1,553,513	1,570,215
Commercial non-real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	1,431,706	1,424,357
Commercial non-real estate | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	28,859	103,514
Commercial non-real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	92,849	42,048
Commercial non-real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	99	296
Commercial non-real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	788,392	777,012
Loans covered by a FDIC loss sharing agreement	29,691	31,891
Total	818,083	808,903
Residential real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	776,696	763,797
Residential real estate | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	986	6,297
Residential real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	10,694	6,863
Residential real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	16	55
Residential real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	52,511	51,758
Loans covered by a FDIC loss sharing agreement	0	0
Total	52,511	51,758
Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	51,657	50,796
Consumer | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	728	755
Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	122	205
Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	4	2
Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	39,908	47,541
Loans covered by a FDIC loss sharing agreement	0	0
Total	39,908	47,541
Other | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	39,908	47,541
Other | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Other | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Other | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	0	0
Other | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Total financing receivables	$ 0	$ 0