Loans - Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	$ 7,491,586	$ 6,116,306
Loans covered by FDIC loss share agreements	73,272	97,030
Total	7,564,858	6,213,336
Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	6,921,071	5,620,817
Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	327,608	310,378
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	241,605	183,643
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,302	1,468
Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	934,980	817,767
Loans covered by FDIC loss share agreements	73,272	97,030
Total	1,008,252	914,797
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	919,224	800,677
Residential real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	4,741	4,890
Residential real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	10,885	11,877
Residential real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	130	323
Residential real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,414,758	2,508,109
Loans covered by FDIC loss share agreements	0	0
Total	3,414,758	2,508,109
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,276,048	2,384,980
Commercial real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	81,148	66,024
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	57,415	56,905
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	147	200
Commercial real estate | 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	1,173,910	1,163,497
Loans covered by FDIC loss share agreements	0	0
Total	1,173,910	1,163,497
Commercial non real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,093,913	1,051,772
Commercial non real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	37,283	50,242
Commercial non real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	42,319	60,801
Commercial non real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	395	682
Commercial non real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Agriculture
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,849,869	1,515,549
Loans covered by FDIC loss share agreements	0	0
Total	1,849,869	1,515,549
Agriculture | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,514,344	1,272,312
Agriculture | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	204,326	189,144
Agriculture | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	130,569	53,837
Agriculture | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	630	256
Agriculture | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	75,592	73,013
Loans covered by FDIC loss share agreements	0	0
Total	75,592	73,013
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	75,065	72,705
Consumer | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	110	78
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	417	223
Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	7
Consumer | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	42,477	38,371
Loans covered by FDIC loss share agreements	0	0
Total	42,477	38,371
Other | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	42,477	38,371
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