Loans - Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 90,287	$ 56,155
Total financing receivables	7,383,120	6,116,307
Loans covered by FDIC loss share agreements	80,137	97,030
Total	7,463,257	6,213,337
Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	6,748,836	5,620,818
Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	395,893	310,378
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	237,610	183,643
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	781	1,468
Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	5,834	7,642
Total financing receivables	937,523	816,449
Loans covered by FDIC loss share agreements	80,137	97,030
Total	1,017,660	913,479
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	921,503	799,359
Residential real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	5,212	4,890
Residential real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	10,705	11,877
Residential real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	103	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]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	10,331	9,556
Total financing receivables	3,298,954	2,508,109
Loans covered by FDIC loss share agreements	0	0
Total	3,298,954	2,508,109
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,128,241	2,384,980
Commercial real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	108,694	66,024
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	61,864	56,905
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	155	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]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	11,678	14,281
Total financing receivables	1,248,378	1,164,816
Loans covered by FDIC loss share agreements	0	0
Total	1,248,378	1,164,816
Commercial non real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,148,620	1,053,091
Commercial non real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	69,731	50,242
Commercial non real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	29,504	60,801
Commercial non real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	523	682
Commercial non real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Agriculture
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	62,356	24,569
Total financing receivables	1,773,821	1,515,549
Loans covered by FDIC loss share agreements	0	0
Total	1,773,821	1,515,549
Agriculture | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,426,550	1,272,312
Agriculture | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	212,049	189,144
Agriculture | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	135,222	53,837
Agriculture | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	256
Agriculture | Loss
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Consumer
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	88	107
Total financing receivables	79,000	73,013
Loans covered by FDIC loss share agreements	0	0
Total	79,000	73,013
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	78,478	72,705
Consumer | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	207	78
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	315	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	45,444	38,371
Loans covered by FDIC loss share agreements	0	0
Total	45,444	38,371
Other | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	45,444	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