Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	$ 8,401,520	$ 8,315,566
Financing receivables, past due	19,144	6,286
Nonaccrual loans	129,407	114,998
Total Loans	8,550,071	8,436,850
Nonaccrual loans with no associated allowance	58,100	44,200
Interest received on nonaccrual loans	1,100	4,400
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	6,154,416	6,055,154
Financing receivables, past due	19,144	6,286
Nonaccrual loans	129,407	114,998
Total Loans	6,302,967	6,176,438
Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, not past due	2,247,104	2,260,412
Financing receivables, past due	0	0
Nonaccrual loans	0	0
Total Loans	2,247,104	2,260,412
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	4,733	4,582
30-59 Days \| Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	4,733	4,582
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	261	632
60-89 Days \| Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	261	632
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	14,150	1,072
90 Days and Greater \| Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	14,150	1,072
90 Days and Greater \| Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivables, past due	$ 0	$ 0

Unpaid principal balance of loans before unamortized premiums, discounts and other cost basis adjustments

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