Loans - Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	$ 9,448,197	$ 8,874,149
Loans covered by FDIC loss share agreement	0	31,891
Total	9,448,197	8,906,040
Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	7,789,080	8,066,457
Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	945,195	375,787
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	685,959	425,685
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	27,963	6,220
Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	4,873,003	4,573,084
Loans covered by FDIC loss share agreement	0	0
Total	4,873,003	4,573,084
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	4,062,814	4,433,530
Commercial real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	577,399	85,256
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	229,467	54,242
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,323	56
Commercial real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Agriculture
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,595,039	1,854,539
Loans covered by FDIC loss share agreement	0	0
Total	1,595,039	1,854,539
Agriculture | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	968,875	1,346,436
Agriculture | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	265,714	179,965
Agriculture | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	348,910	322,327
Agriculture | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	11,540	5,811
Agriculture | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Commercial non-real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	2,051,663	1,570,215
Loans covered by FDIC loss share agreement	0	0
Total	2,051,663	1,570,215
Commercial non-real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,851,323	1,424,357
Commercial non-real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	94,401	103,514
Commercial non-real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	94,316	42,048
Commercial non-real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	11,623	296
Commercial non-real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	828,054	777,012
Loans covered by FDIC loss share agreement	0	31,891
Total	828,054	808,903
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	806,436	763,797
Residential real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	6,972	6,297
Residential real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	13,173	6,863
Residential real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,473	55
Residential real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	63,091	51,758
Loans covered by FDIC loss share agreement	0	0
Total	63,091	51,758
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	62,285	50,796
Consumer | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	709	755
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	93	205
Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	4	2
Consumer | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	37,347	47,541
Loans covered by FDIC loss share agreement	0	0
Total	37,347	47,541
Other | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	37,347	47,541
Other | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	$ 0	$ 0