Fair Value Level 3 Valuation - Nonrecurring (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	$ 6,490	$ 7,825
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	116	229
Mortgage loans held for investment, at fair value	3,512,672	3,270,535
Total nonrecurring assets at fair value	3,578,668	3,313,691
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	5,502	7,054
Mortgage loans held for investment, at fair value	255,556	127,650
Total nonrecurring assets at fair value	262,442	136,048
Fair Value, Recurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	6,490	7,825
Total nonrecurring assets at fair value	147,074	58,623
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	5,629	7,137
Total nonrecurring assets at fair value	14,417	18,685
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	861	688
Total nonrecurring assets at fair value	1,986	1,725
Fair Value, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	25	274
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	1,087	1,076
Total nonrecurring assets at fair value	2,742	2,598
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consensus without inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	754	471
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single vendor without inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	333	605
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_MultifamilyMember
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	390	40
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_MultifamilyMember | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	40	16
Acquired property, net	25	9
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_MultifamilyMember | Internal model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	125
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_MultifamilyMember | Asset Manager Estimate [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	0	24
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_MultifamilyMember | Appraisals [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	225
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_SingleFamily
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	261	918
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_SingleFamily | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	11	51
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_SingleFamily | Internal model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	979	555
Acquired property, net	41	164
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_SingleFamily | Accepted Offers [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	35	101
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_SingleFamily | Appraisals [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	89	362
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | FNM_SingleFamily | Walk Forwards [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	$ 85	$ 240