Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	$ 6,996,039
Financing receivables, past due	29,997
Nonaccrual loans	122,072	$ 87,781
Gross financing receivables	7,148,108
Nonaccrual loans with no associated allowance	24,000
Interest received on nonaccrual loans	1,000
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	5,206,313
Financing receivables, past due	29,997
Nonaccrual loans	122,072
Gross financing receivables	5,358,382
Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	1,789,726
Financing receivables, past due	0
Nonaccrual loans	0
Gross financing receivables	1,789,726
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	4,821
30-59 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	4,821
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	3,024
60-89 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	3,024
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	22,152
90 Days and Greater | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	22,152
90 Days and Greater | Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	$ 0