Fair Value Fair Value Option (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage loans held for investment, at fair value	$ 12,914	$ 14,075
Fair value of nonaccrual loans	159	238
Difference between unpaid principal balance and the fair value of nonaccrual loans	32	59
Fair value of loans that are 90 days or more past due	156	256
Difference between unpaid principal balance and the fair value of these 90 days or more past due loans	$ 26	$ 52
Single-family [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Serious delinquency, days past due	90 days	90 days
Fannie Mae [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt, fair value	$ 10,460	$ 11,133
Consolidated Trusts [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt, fair value	35,453	23,609
Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, unpaid principal balance	12,173	13,661
Long-term debt [Member] | Fannie Mae [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt, unpaid principal balance	9,924	11,263
Long-term debt [Member] | Consolidated Trusts [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Long-term debt, unpaid principal balance	$ 31,633	$ 21,604