Summary of Significant Accounting Policies - CECL Adoption (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2020	Jan. 01, 2020	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
Total Allowance		$ 12,618
Long-term standby purchase commitments and guarantees	$ 3,020	2,164
Securities borrowed, allowance for credit loss		0
Retained earnings	824,116	799,276	$ 693,617		$ 773,742	$ 763,298		$ 752,557
Allowance for losses	14,939	10,454
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities borrowed, allowance for credit loss				$ 9
Retained earnings		(2,099)
Allowance for losses				2,656
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities borrowed, allowance for credit loss				9
Retained earnings		797,177
Allowance for losses				15,274
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	478,647	457,047	455,545		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
Total Allowance		$ 10,454
Long-term standby purchase commitments and guarantees	1,650	2,164	2,020	2,016	1,880	2,038	$ 2,167	2,167
Allowance for losses	6,039	10,454	7,353
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
Allowance for losses		(3,909)	0	(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
Allowance for losses		$ 6,545	7,353	6,545
Rural  Utilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Total Allowance		$ 0
Long-term standby purchase commitments and guarantees	1,370	0	1,400	1,011	$ 0	0	0	$ 0
Allowance for losses	$ 8,900	0	7,503
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
Allowance for losses		5,378	0	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
Allowance for losses		5,378	$ 7,503	5,378
Institutional Credit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total Allowance		$ 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