Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	$ 7,796,712	$ 7,796,712	$ 7,261,933
Nonaccrual loans	118,294	118,294	114,998
Total Loans	8,847,773	8,847,773	8,436,850
Nonaccrual loans with no associated allowance	58,400	58,400	44,200
Interest received on nonaccrual loans	1,900	3,000	4,400
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	6,601,205	6,601,205	6,176,438
Nonaccrual loans	118,294	118,294	114,998
Total Loans	6,601,205	6,601,205	6,176,438
Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,246,568	2,246,568	2,260,412
Nonaccrual loans	0	0	0
Total Loans	2,246,568	2,246,568	2,260,412
Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	8,712,035	8,712,035	8,315,566
Current | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	6,475,467	6,475,467	6,055,154
Current | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,236,568	2,236,568	2,260,412
Total Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	17,444	17,444	6,286
Total Past Due | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	7,444	7,444	6,286
Total Past Due | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	10,000	10,000	0
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	11,501	11,501	4,582
30-59 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	1,501	1,501	4,582
30-59 Days | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	10,000	10,000	0
60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	525	525	632
60-89 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	525	525	632
60-89 Days | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	0	0	0
90 Days and Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	5,418	5,418	1,072
90 Days and Greater | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	5,418	5,418	1,072
90 Days and Greater | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	$ 0	$ 0	$ 0