Allowance for Loan and Lease Losses - Summary of Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	$ 135,950	$ 68,003	$ 70,774	$ 64,540
Charge-offs	(10,607)	(17,969)	(27,640)	(28,699)
Recoveries	1,174	435	3,485	1,740
Provision	21,601	26,200	101,351	39,206
(Improvement) impairment of ASC 310-30 loans	40	(123)	188	(241)
Allowance for credit losses, ending balance	148,158	76,546	148,158	76,546
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	64,414	15,603	16,827	16,777
Charge-offs	(1,335)	(45)	(2,789)	(990)
Recoveries	121	169	355	428
Provision	15,392	1,606	64,249	1,537
(Improvement) impairment of ASC 310-30 loans	31	(9)	(19)	(428)
Allowance for credit losses, ending balance	78,623	17,324	78,623	17,324
Agriculture
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	29,526	33,725	30,819	28,121
Charge-offs	(4,250)	(12,759)	(13,378)	(19,704)
Recoveries	735	0	2,143	274
Provision	3,418	15,159	10,110	27,434
(Improvement) impairment of ASC 310-30 loans	0	0	(265)	0
Allowance for credit losses, ending balance	29,429	36,125	29,429	36,125
Commercial non-real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	31,766	13,049	17,567	13,610
Charge-offs	(3,210)	(4,608)	(8,269)	(6,079)
Recoveries	131	79	303	306
Provision	359	8,567	19,445	9,250
(Improvement) impairment of ASC 310-30 loans	0	49	0	49
Allowance for credit losses, ending balance	29,046	17,136	29,046	17,136
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	8,356	4,326	4,095	4,749
Charge-offs	(92)	(182)	(379)	(824)
Recoveries	48	48	360	335
Provision	229	451	4,023	82
(Improvement) impairment of ASC 310-30 loans	0	(163)	442	138
Allowance for credit losses, ending balance	8,541	4,480	8,541	4,480
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	777	374	427	257
Charge-offs	(20)	(18)	(65)	(351)
Recoveries	16	15	64	143
Provision	138	150	464	472
(Improvement) impairment of ASC 310-30 loans	9	0	30	0
Allowance for credit losses, ending balance	920	521	920	521
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning balance	1,111	926	1,039	1,026
Charge-offs	(1,700)	(357)	(2,760)	(751)
Recoveries	123	124	260	254
Provision	2,065	267	3,060	431
(Improvement) impairment of ASC 310-30 loans	0	0	0	0
Allowance for credit losses, ending balance	$ 1,599	$ 960	$ 1,599	$ 960