Loans - Schedule of the Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 9,611,891	$ 8,874,149
Loans covered by a FDIC loss sharing agreement	0	31,891
Total	9,611,891	8,906,040
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	8,506,615	8,066,457
Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	470,580	375,787
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	630,437	425,685
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,259	6,220
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,893,518	4,573,084
Loans covered by a FDIC loss sharing agreement	0	0
Total	4,893,518	4,573,084
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,544,252	4,433,530
Commercial real estate | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	175,893	85,256
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	173,324	54,242
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	49	56
Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Agriculture
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,676,720	1,854,539
Loans covered by a FDIC loss sharing agreement	0	0
Total	1,676,720	1,854,539
Agriculture | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,095,048	1,346,436
Agriculture | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	234,251	179,965
Agriculture | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	344,027	322,327
Agriculture | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,394	5,811
Agriculture | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Commercial non-real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	2,085,095	1,570,215
Loans covered by a FDIC loss sharing agreement	0	0
Total	2,085,095	1,570,215
Commercial non-real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,936,134	1,424,357
Commercial non-real estate | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	48,334	103,514
Commercial non-real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	99,828	42,048
Commercial non-real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	799	296
Commercial non-real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	860,518	777,012
Loans covered by a FDIC loss sharing agreement	0	31,891
Total	860,518	808,903
Residential real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	835,956	763,797
Residential real estate | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	11,392	6,297
Residential real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	13,156	6,863
Residential real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	14	55
Residential real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	61,327	51,758
Loans covered by a FDIC loss sharing agreement	0	0
Total	61,327	51,758
Consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	60,512	50,796
Consumer | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	710	755
Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	102	205
Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3	2
Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Other
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	34,713	47,541
Loans covered by a FDIC loss sharing agreement	0	0
Total	34,713	47,541
Other | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	34,713	47,541
Other | Watchlist
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Other | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Other | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Other | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 0	$ 0