Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	$ 8,315,566
Financing receivables, past due	6,286
Nonaccrual loans	114,998	$ 87,781
Total Loans	8,436,850
Nonaccrual loans with no associated allowance	44,200
Interest received on nonaccrual loans	4,400
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	6,055,154
Financing receivables, past due	6,286
Nonaccrual loans	114,998
Total Loans	6,176,438
Rural Utilities:
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	2,260,412
Financing receivables, past due	0
Nonaccrual loans	0
Total Loans	2,260,412
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	4,582
30-59 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	4,582
30-59 Days | Rural Utilities:
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	0
60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	632
60-89 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	632
60-89 Days | Rural Utilities:
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	0
90 Days and Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	1,072
90 Days and Greater | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	1,072	$ 57,719
90 Days and Greater | Farm & Ranch | Payment Deferral, CARES Act
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	145,500
90 Days and Greater | Rural Utilities:
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	$ 0