Loans - Schedule of Composition of Loan Balances (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	$ 8,847,773		$ 8,436,850
Total	7,796,712		7,261,933
Allowance for losses	(14,000)		(13,832)
Total loans, net of allowance	8,860,705		8,535,146
Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	8,847,773		8,436,850
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	26,932		112,128
Total	8,874,705		8,548,978
Allowance for losses	(14,000)		(13,832)
Total loans, net of allowance	8,860,705		8,535,146
Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	6,601,205		6,176,438
Total	6,601,205		6,176,438
Allowance for losses	(3,092)	$ (3,718)	(3,745)	$ (6,039)	$ (7,353)	$ (10,454)
Farm & Ranch | Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	6,601,205		6,176,438
Rural Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,246,568		2,260,412
Total	2,246,568		2,260,412
Allowance for losses	(10,908)	$ (11,089)	(10,087)	$ (8,900)	$ (7,503)	$ 0
Rural Utilities | Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,246,568		2,260,412
Unsecuritized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	7,769,780		7,149,805
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	26,932		112,128
Total	7,796,712		7,261,933
Allowance for losses	(13,290)		(12,943)
Total loans, net of allowance	7,783,422		7,248,990
Unsecuritized | Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	5,523,212		4,889,393
Unsecuritized | Rural Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,246,568		2,260,412
In Consolidated Trusts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	1,077,993		1,287,045
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	0		0
Total	1,077,993		1,287,045
Allowance for losses	(710)		(889)
Total loans, net of allowance	1,077,283		1,286,156
In Consolidated Trusts | Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	1,077,993		1,287,045
In Consolidated Trusts | Rural Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	$ 0		$ 0