Summary of Significant Accounting Policies - CECL Adoption (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loans receivable			$ 12,618
Long-term standby purchase commitments and guarantees	$ 3,277		2,164
Transition Adjustment	13,832		10,454
Investment Securities			0
Retained earnings	992,477		799,276	$ 752,557	$ 708,644
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	509,560		457,047	393,351	323,175
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			(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]
Allowance for loans receivable			10,454
Long-term standby purchase commitments and guarantees	2,097	2,016	2,164	2,167	2,070
Transition Adjustment	3,745		10,454	7,017	6,796
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		(148)	(148)
Transition Adjustment		(3,909)	(3,909)
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	6,545
Rural Utilities:
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loans receivable			0
Long-term standby purchase commitments and guarantees	1,180	1,011	0	0	0
Transition Adjustment	$ 10,087		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		1,011	1,011
Transition Adjustment		5,378	5,378
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	5,378
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