Loans - Schedule of Composition of Loan Balances (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	$ 7,966,679
Total	6,825,061		$ 5,390,977
Allowance for losses	(15,821)		(10,454)
Total loans, net of allowance	8,105,647		6,981,440
Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	7,966,679		6,947,850
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	154,789		44,044
Total	8,121,468		6,991,894
Allowance for losses	(15,821)		(10,454)
Total loans, net of allowance	8,105,647		6,981,440
Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	5,857,324
Total	5,857,324
Allowance for losses	(5,739)	$ (6,039)	(10,454)	$ (8,024)	$ (7,264)	$ (7,017)
Farm & Ranch | Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	5,857,324		5,276,557
Rural  Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,109,355
Total	2,109,355
Allowance for losses	(10,082)	$ (8,900)	0	$ 0	$ 0	$ 0
Rural  Utilities | Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,109,355		1,671,293
Unsecuritized
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	6,690,272		5,346,933
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	154,789		44,044
Total	6,845,061		5,390,977
Allowance for losses	(14,878)		(8,853)
Total loans, net of allowance	6,830,183		5,382,124
Unsecuritized | Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	4,580,917		3,675,640
Unsecuritized | Rural  Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	2,109,355		1,671,293
In Consolidated Trusts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	1,276,407		1,600,917
Unamortized premiums, discounts, fair value hedge basis adjustment, and other cost basis adjustments	0		0
Total	1,276,407		1,600,917
Allowance for losses	(943)		(1,601)
Total loans, net of allowance	1,275,464		1,599,316
In Consolidated Trusts | Farm & Ranch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	1,276,407		1,600,917
In Consolidated Trusts | Rural  Utilities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid principal balance	$ 0		$ 0