Related Party Transactions - Schedule of Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Mortgages | Rural Utilities
Related Party Transaction [Line Items]
Purchases:	$ 776,400
Zions Bancorporation, National Association | Sale of Farmer Mac Guaranteed Securities
Related Party Transaction [Line Items]
Sales of Farmer Mac Guaranteed Securities	163,134	$ 68,721	$ 128,924
Zions Bancorporation, National Association | USDA Guarantee | USDA Securities:
Related Party Transaction [Line Items]
Purchases:	8,875	19,120	20,368
Zions Bancorporation, National Association | Mortgages | Farm & Ranch
Related Party Transaction [Line Items]
Purchases:	129,040	114,719	126,449
CFC
Related Party Transaction [Line Items]
Purchases:	660,000	986,645	487,341
CFC | Rural Utilities | Guarantee Obligations
Related Party Transaction [Line Items]
Purchases:	575,000	675,000	350,000
CFC | Institutional Credit | US Government-sponsored Enterprises Debt Securities | Off-Balance Sheet:
Related Party Transaction [Line Items]
Purchases:	0	300,000	0
CFC | Real Estate Loan | Rural Utilities
Related Party Transaction [Line Items]
Purchases:	$ 85,000	$ 11,645	$ 137,341