Loans - Unpaid Principal Balances by Delinquency Status (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	$ 7,758,286	$ 7,758,286	$ 7,261,933
Nonaccrual loans	137,924	137,924	114,998
Total Loans	9,020,921	9,020,921	8,436,850
Nonaccrual loans with no associated allowance	44,800	44,800	44,200
Interest received on nonaccrual loans	1,400	4,400	4,400
Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	6,777,749	6,777,749	6,176,438
Nonaccrual loans	137,924	137,924	114,998
Total Loans	6,777,749	6,777,749	6,176,438
Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,243,172	2,243,172	2,260,412
Nonaccrual loans	0	0	0
Total Loans	2,243,172	2,243,172	2,260,412
Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	8,876,515	8,876,515	8,315,566
Current | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	6,633,343	6,633,343	6,055,154
Current | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,243,172	2,243,172	2,260,412
Total Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	6,482	6,482	6,286
Total Past Due | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	6,482	6,482	6,286
Total Past Due | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	0	0	0
30-59 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,345	2,345	4,582
30-59 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	2,345	2,345	4,582
30-59 Days | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	0	0	0
60-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	482	482	632
60-89 Days | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	482	482	632
60-89 Days | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	0	0	0
90 Days and Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	3,655	3,655	1,072
90 Days and Greater | Farm & Ranch
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	3,655	3,655	1,072
90 Days and Greater | Rural Utilities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total	$ 0	$ 0	$ 0