Short-Term and Long-Term Debt Short-Term Debt (Details) - Fannie Mae [Member] - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Short-Term Debt [Line Items]
Short-term debt, weighted-average interest rate	0.06%	0.18%
Short-term debt, outstanding	$ 2,857	$ 12,173

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant