Summary of Significant Accounting Policies - CECL Adoption (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Allowance for loans receivable		$ 12,618
Long-term standby purchase commitments and guarantees	$ 3,568	2,164
Transition Adjustment	15,821	10,454
Investment Securities		0
Retained earnings	930,345	799,276	$ 824,116	$ 693,617		$ 750,259	$ 773,742	$ 763,298		$ 752,557
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transition Adjustment					$ 2,656
Investment Securities					9
Retained earnings		(2,099)
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transition Adjustment					15,274
Investment Securities					9
Retained earnings		797,177
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 488,717	457,047	478,647	$ 455,545		435,479	428,569	$ 407,755		393,351
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		(2,099)			(2,099)
Retained Earnings | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		454,948			454,948
Farm & Ranch
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Allowance for loans receivable		10,454
Long-term standby purchase commitments and guarantees	$ 2,278	2,164	1,650		2,016	1,743	1,880		$ 2,167	2,167
Transition Adjustment	$ 5,739	10,454	6,039			8,024	7,264			7,017
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Long-term standby purchase commitments and guarantees			0		(148)		0		0
Transition Adjustment		(3,909)	0		(3,909)		0			0
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Long-term standby purchase commitments and guarantees					2,016
Transition Adjustment		6,545	6,039		6,545		7,264			7,017
Rural  Utilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Allowance for loans receivable		0
Long-term standby purchase commitments and guarantees	$ 1,290	0	1,370		1,011	0	0		0	0
Transition Adjustment	$ 10,082	0	8,900			$ 0	0			0
Rural  Utilities | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Long-term standby purchase commitments and guarantees			0		1,011		0		$ 0
Transition Adjustment		5,378	0		5,378		0			0
Rural  Utilities | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Long-term standby purchase commitments and guarantees					1,011
Transition Adjustment		5,378	$ 8,900		5,378		$ 0			$ 0
AgVantage Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loans receivable		$ 0
AgVantage Securities | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transition Adjustment					315
AgVantage Securities | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transition Adjustment					$ 315