Loans - Schedule of Past Due Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 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
Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,716,122	4,573,084
Loans covered by a FDIC loss sharing agreement	0	0
Total	4,716,122	4,573,084
Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,734,388	1,854,539
Loans covered by a FDIC loss sharing agreement	0	0
Total	1,734,388	1,854,539
Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,553,513	1,570,215
Loans covered by a FDIC loss sharing agreement	0	0
Total	1,553,513	1,570,215
Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	788,392	777,012
Loans covered by a FDIC loss sharing agreement	29,691	31,891
Total	818,083	808,903
Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	52,511	51,758
Loans covered by a FDIC loss sharing agreement	0	0
Total	52,511	51,758
Other
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	39,908	47,541
Loans covered by a FDIC loss sharing agreement	0	0
Total	39,908	47,541
Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	174,262	64,979
Loans covered by a FDIC loss sharing agreement	2,277	1,277
Total	176,539	66,256
Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	43,159	6,632
Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	100,532	47,767
Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	23,991	8,476
Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	6,504	1,926
Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	76	178
Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Past Due | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	61,829	21,302
Loans covered by a FDIC loss sharing agreement	1,201	536
Total	63,030	21,838
Past Due | 30-59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	25,351	3,587
Past Due | 30-59 Days Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	30,196	13,411
Past Due | 30-59 Days Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	2,538	3,932
Past Due | 30-59 Days Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,702	311
Past Due | 30-59 Days Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	42	61
Past Due | 30-59 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Past Due | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	15,429	2,743
Loans covered by a FDIC loss sharing agreement	319	410
Total	15,748	3,153
Past Due | 60-89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,056	570
Past Due | 60-89 Days Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	10,160	1,267
Past Due | 60-89 Days Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	956	120
Past Due | 60-89 Days Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	252	676
Past Due | 60-89 Days Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	5	110
Past Due | 60-89 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Past Due | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	97,004	40,934
Loans covered by a FDIC loss sharing agreement	757	331
Total	97,761	41,265
Past Due | 90 Days or Greater Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	13,752	2,475
Past Due | 90 Days or Greater Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	60,176	33,089
Past Due | 90 Days or Greater Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	20,497	4,424
Past Due | 90 Days or Greater Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	2,550	939
Past Due | 90 Days or Greater Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	29	7
Past Due | 90 Days or Greater Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Current
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	8,710,572	8,809,170
Loans covered by a FDIC loss sharing agreement	27,414	30,614
Total	8,737,986	8,839,784
Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,672,963	4,566,452
Current | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,633,856	1,806,772
Current | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,529,522	1,561,739
Current | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	781,888	775,086
Current | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	52,435	51,580
Current | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 39,908	$ 47,541