Loans - Schedule of Composition of Loan Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	$ 8,436,850
Total	7,261,933	$ 5,390,977
Allowance for losses	(13,832)	(10,454)
Total loans, net of allowance	8,535,146	6,981,440
Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	8,436,850	6,947,850
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	112,128	44,044
Total	8,548,978	6,991,894
Allowance for losses	(13,832)	(10,454)
Total loans, net of allowance	8,535,146	6,981,440
Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	6,176,438
Total	6,176,438
Allowance for losses	(3,745)	(10,454)	$ (7,017)	$ (6,796)
Farm & Ranch | Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	6,176,438	5,276,557
Rural Utilities:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,260,412
Total	2,260,412
Allowance for losses	(10,087)	0	$ 0	$ 0
Rural Utilities: | Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,260,412	1,671,293
Unsecuritized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	7,149,805	5,346,933
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	112,128	44,044
Total	7,261,933	5,390,977
Allowance for losses	(12,943)	(8,853)
Total loans, net of allowance	7,248,990	5,382,124
Unsecuritized | Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	4,889,393	3,675,640
Unsecuritized | Rural Utilities:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,260,412	1,671,293
In Consolidated Trusts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	1,287,045	1,600,917
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	0	0
Total	1,287,045	1,600,917
Allowance for losses	(889)	(1,601)
Total loans, net of allowance	1,286,156	1,599,316
In Consolidated Trusts | Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	1,287,045	1,600,917
In Consolidated Trusts | Rural Utilities:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	$ 0	$ 0