Loans - Schedule of Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016
With an allowance recorded:
Recorded Investment	$ 134,610		$ 134,610		$ 142,619
Unpaid Principal Balance	154,488		154,488		155,753
Related Allowance	24,847		24,847		25,686
With no allowance recorded:
Recorded Investment	94,023		94,023		73,810
Unpaid Principal Balance	99,914		99,914		78,855
Total Recorded Investment, Impaired Loans	228,633		228,633		216,429
Total Unpaid Principal Balance, Impaired Loans	254,402		254,402		234,608
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	234,141	$ 249,046	228,236	$ 246,397
Interest Income Recognized While on Impaired Status	3,028	3,651	8,483	10,190
Residential real estate
With an allowance recorded:
Recorded Investment	6,029		6,029		6,244
Unpaid Principal Balance	6,764		6,764		6,886
Related Allowance	3,134		3,134		3,000
With no allowance recorded:
Recorded Investment	2,690		2,690		4,120
Unpaid Principal Balance	4,517		4,517		5,807
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	9,051	11,463	9,489	11,743
Interest Income Recognized While on Impaired Status	112	147	352	439
Commercial real estate
With an allowance recorded:
Recorded Investment	18,795		18,795		29,965
Unpaid Principal Balance	22,862		22,862		32,349
Related Allowance	4,759		4,759		3,846
With no allowance recorded:
Recorded Investment	23,509		23,509		24,040
Unpaid Principal Balance	24,483		24,483		24,660
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	40,939	77,907	45,294	78,962
Interest Income Recognized While on Impaired Status	579	1,000	1,794	3,192
Commercial non real estate
With an allowance recorded:
Recorded Investment	26,422		26,422		34,526
Unpaid Principal Balance	32,898		32,898		35,283
Related Allowance	6,568		6,568		6,475
With no allowance recorded:
Recorded Investment	14,994		14,994		15,299
Unpaid Principal Balance	16,279		16,279		16,469
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	43,224	32,560	45,424	37,322
Interest Income Recognized While on Impaired Status	331	388	1,111	1,162
Agriculture
With an allowance recorded:
Recorded Investment	82,982		82,982		71,501
Unpaid Principal Balance	91,567		91,567		80,842
Related Allowance	10,275		10,275		12,278
With no allowance recorded:
Recorded Investment	52,808		52,808		30,339
Unpaid Principal Balance	54,611		54,611		31,907
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	140,512	126,806	127,621	118,056
Interest Income Recognized While on Impaired Status	1,993	2,103	5,186	5,359
Consumer
With an allowance recorded:
Recorded Investment	382		382		383
Unpaid Principal Balance	397		397		393
Related Allowance	111		111		87
With no allowance recorded:
Recorded Investment	22		22		12
Unpaid Principal Balance	24		24		$ 12
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	415	310	408	314
Interest Income Recognized While on Impaired Status	$ 13	$ 13	$ 40	$ 38