Loans - Schedule of Past Due Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 7,645,722	$ 7,491,586
Loans covered by FDIC loss sharing agreements	68,553	73,272
Total	7,714,275	7,564,858
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	927,594	934,980
Loans covered by FDIC loss sharing agreements	68,553	73,272
Total	996,147	1,008,252
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,551,434	3,414,758
Loans covered by FDIC loss sharing agreements	0	0
Total	3,551,434	3,414,758
Commercial non real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,138,286	1,173,910
Loans covered by FDIC loss sharing agreements	0	0
Total	1,138,286	1,173,910
Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,909,645	1,849,869
Loans covered by FDIC loss sharing agreements	0	0
Total	1,909,645	1,849,869
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	71,194	75,592
Loans covered by FDIC loss sharing agreements	0	0
Total	71,194	75,592
Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	47,569	42,477
Loans covered by FDIC loss sharing agreements	0	0
Total	47,569	42,477
Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	49,166	40,094
Loans covered by FDIC loss sharing agreements	2,020	2,944
Total	51,186	43,038
Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	4,163	3,439
Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	10,659	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	18,983	16,697
Past Due | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	15,122	12,232
Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	239	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	14,372	4,364
Loans covered by FDIC loss sharing agreements	1,042	1,404
Total	15,414	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	1,866	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	4,839	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	1,105	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	6,378	(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	184	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	6,563	8,751
Loans covered by FDIC loss sharing agreements	512	1,173
Total	7,075	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	271	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	454	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	5,668	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	146	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	24	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	28,231	26,979
Loans covered by FDIC loss sharing agreements	466	367
Total	28,697	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	2,026	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	5,366	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	12,210	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	8,598	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	31	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	7,596,556	7,451,492
Loans covered by FDIC loss sharing agreements	66,533	70,328
Total	7,663,089	7,521,820
Current | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	923,431	931,541
Current | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	3,540,775	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	1,119,303	1,157,213
Current | Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	1,894,523	1,837,637
Current | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	70,955	75,335
Current | Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, net of unamortized discount on acquired loans	$ 47,569	$ 42,477