Summary of Significant Accounting Policies - CECL Adoption (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses	$ 14,856		$ 10,454
Long-term standby purchase commitments and guarantees	3,420		2,164
Securities borrowed, allowance for credit loss			0
Total Allowance			12,618
Retained earnings	455,545		457,047
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities borrowed, allowance for credit loss		$ 9
Total Allowance		2,656
Retained earnings		(2,099)
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities borrowed, allowance for credit loss		9
Total Allowance		15,274
Retained earnings		454,948
Farm & Ranch
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses	7,353		10,454
Long-term standby purchase commitments and guarantees	2,020	2,016	2,164	$ 2,038	$ 2,167	$ 2,167
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses		(3,909)
Long-term standby purchase commitments and guarantees		(148)
Farm & Ranch | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses		6,545
Long-term standby purchase commitments and guarantees		2,016
Rural  Utilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses	7,503		0
Long-term standby purchase commitments and guarantees	$ 1,400	1,011	0	$ 0	$ 0	$ 0
Rural  Utilities | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses		5,378
Long-term standby purchase commitments and guarantees		1,011
Rural  Utilities | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses		5,378
Long-term standby purchase commitments and guarantees		1,011
Institutional Credit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses			$ 0
Institutional Credit | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses		315
Institutional Credit | Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for losses		$ 315