Loans - Schedule of Past Due Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	$ 238,291	$ 64,979
Financing receivables, not past due	9,373,600	8,809,170
Total financing receivables	9,611,891	8,874,149
Loans covered by a FDIC loss sharing agreement, past due	0	1,277
Loans covered by a FDIC loss sharing agreement, not past due	0	30,614
Loans covered by a FDIC loss sharing agreement	0	31,891
Total, past due	238,291	66,256
Total, not past due	9,373,600	8,839,784
Total	9,611,891	8,906,040
Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	59,169	6,632
Financing receivables, not past due	4,834,349	4,566,452
Total financing receivables	4,893,518	4,573,084
Loans covered by a FDIC loss sharing agreement	0	0
Total	4,893,518	4,573,084
Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	157,669	47,767
Financing receivables, not past due	1,519,051	1,806,772
Total financing receivables	1,676,720	1,854,539
Loans covered by a FDIC loss sharing agreement	0	0
Total	1,676,720	1,854,539
Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	15,939	8,476
Financing receivables, not past due	2,069,156	1,561,739
Total financing receivables	2,085,095	1,570,215
Loans covered by a FDIC loss sharing agreement	0	0
Total	2,085,095	1,570,215
Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	5,466	1,926
Financing receivables, not past due	855,052	775,086
Total financing receivables	860,518	777,012
Loans covered by a FDIC loss sharing agreement	0	31,891
Total	860,518	808,903
Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	48	178
Financing receivables, not past due	61,279	51,580
Total financing receivables	61,327	51,758
Loans covered by a FDIC loss sharing agreement	0	0
Total	61,327	51,758
Other
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	0	0
Financing receivables, not past due	34,713	47,541
Total financing receivables	34,713	47,541
Loans covered by a FDIC loss sharing agreement	0	0
Total	34,713	47,541
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	83,521	21,302
Loans covered by a FDIC loss sharing agreement, past due	0	536
Total, past due	83,521	21,838
30-59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	93	3,587
30-59 Days Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	82,476	13,411
30-59 Days Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	21	3,932
30-59 Days Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	916	311
30-59 Days Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	15	61
30-59 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	0	0
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	37,714	2,743
Loans covered by a FDIC loss sharing agreement, past due	0	410
Total, past due	37,714	3,153
60-89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	30,393	570
60-89 Days Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	5,842	1,267
60-89 Days Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	323	120
60-89 Days Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	1,143	676
60-89 Days Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	13	110
60-89 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	0	0
90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	117,056	40,934
Loans covered by a FDIC loss sharing agreement, past due	0	331
Total, past due	117,056	41,265
90 Days or Greater Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	28,683	2,475
90 Days or Greater Past Due | Agriculture
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	69,351	33,089
90 Days or Greater Past Due | Commercial non-real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	15,595	4,424
90 Days or Greater Past Due | Residential real estate
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	3,407	939
90 Days or Greater Past Due | Consumer
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	20	7
90 Days or Greater Past Due | Other
Financing Receivable, Past Due [Line Items]
Financing receivables, past due	$ 0	$ 0