Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	$ 7,826,189	$ 7,826,189
Financing receivables, past due	11,051	11,051	$ 57,700
Nonaccrual loans	129,439	129,439	87,781
Total Loans	7,966,679	7,966,679
Nonaccrual loans with no associated allowance	24,800	24,800
Interest received on nonaccrual loans	1,200	3,500
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	5,716,834	5,716,834
Financing receivables, past due	11,051	11,051
Nonaccrual loans	129,439	129,439
Total Loans	5,857,324	5,857,324
Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	2,109,355	2,109,355
Financing receivables, past due	0	0
Nonaccrual loans	0	0
Total Loans	2,109,355	2,109,355
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	3,513	3,513
30-59 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	3,513	3,513
30-59 Days | Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	0	0
60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	637	637
60-89 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	637	637
60-89 Days | Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	0	0
90 Days and Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	6,901	6,901
90 Days and Greater | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	6,901	6,901	$ 57,719
90 Days and Greater | Farm & Ranch | Payment Deferral, CARES Act
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	105,300	105,300
90 Days and Greater | Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	$ 0	$ 0