Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	$ 7,578,530	$ 7,578,530
Financing receivables, past due	23,420	23,420
Nonaccrual loans	117,130	117,130	$ 87,781
Total Loans	7,719,080	7,719,080
Nonaccrual loans with no associated allowance	23,100	23,100
Interest received on nonaccrual loans	1,300	2,300
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	5,476,962	5,476,962
Financing receivables, past due	23,420	23,420
Nonaccrual loans	117,130	117,130
Total Loans	5,617,512	5,617,512
Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	2,101,568	2,101,568
Financing receivables, past due	0	0
Nonaccrual loans	0	0
Total Loans	2,101,568	2,101,568
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	7,892	7,892
30-59 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	7,892	7,892
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	0	0
60-89 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	0	0
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	15,528	15,528
90 Days and Greater | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	15,528	15,528
90 Days and Greater | Farm & Ranch | Payment Deferral, CARES Act
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	82,200	82,200
90 Days and Greater | Rural  Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	$ 0	$ 0