Loans - Schedule of Past Due Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 7,691,349	$ 7,491,586
Loans covered by FDIC loss sharing agreements	61,789	73,272
Total	7,753,138	7,564,858
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	880,646	934,980
Loans covered by FDIC loss sharing agreements	61,789	73,272
Total	942,435	1,008,252
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,655,902	3,414,758
Loans covered by FDIC loss sharing agreements	0	0
Total	3,655,902	3,414,758
Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,223,503	1,173,910
Loans covered by FDIC loss sharing agreements	0	0
Total	1,223,503	1,173,910
Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,817,619	1,849,869
Loans covered by FDIC loss sharing agreements	0	0
Total	1,817,619	1,849,869
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	69,568	75,592
Loans covered by FDIC loss sharing agreements	0	0
Total	69,568	75,592
Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	44,111	42,477
Loans covered by FDIC loss sharing agreements	0	0
Total	44,111	42,477
Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	60,961	40,094
Loans covered by FDIC loss sharing agreements	1,320	2,944
Total	62,281	43,038
Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,932	3,439
Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	7,983	7,469
Past Due | Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	23,245	16,697
Past Due | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	25,606	12,232
Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	195	257
Past Due | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Past Due | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	14,721	4,364
Loans covered by FDIC loss sharing agreements	190	1,404
Total	14,911	5,768
Past Due | 30-59 Days Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,004	828
Past Due | 30-59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	361	1,765
Past Due | 30-59 Days Past Due | Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,735	1,588
Past Due | 30-59 Days Past Due | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	11,501	(26)
Past Due | 30-59 Days Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	120	209
Past Due | 30-59 Days Past Due | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Past Due | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,668	8,751
Loans covered by FDIC loss sharing agreements	402	1,173
Total	5,070	9,924
Past Due | 60-89 Days Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	606	548
Past Due | 60-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,603	1,959
Past Due | 60-89 Days Past Due | Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	502	5,515
Past Due | 60-89 Days Past Due | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,924	709
Past Due | 60-89 Days Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	33	20
Past Due | 60-89 Days Past Due | Other
Financing Receivable, Recorded Investment, 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, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	41,572	26,979
Loans covered by FDIC loss sharing agreements	728	367
Total	42,300	27,346
Past Due | 90 Days or Greater Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	2,322	2,063
Past Due | 90 Days or Greater Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	6,019	3,745
Past Due | 90 Days or Greater Past Due | Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	21,008	9,594
Past Due | 90 Days or Greater Past Due | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	12,181	11,549
Past Due | 90 Days or Greater Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	42	28
Past Due | 90 Days or Greater Past Due | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	0	0
Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	7,630,388	7,451,492
Loans covered by FDIC loss sharing agreements	60,469	70,328
Total	7,690,857	7,521,820
Current | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	876,714	931,541
Current | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,647,919	3,407,289
Current | Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,200,258	1,157,213
Current | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,792,013	1,837,637
Current | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	69,373	75,335
Current | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 44,111	$ 42,477