Fair Value Level 3 Valuation - Nonrecurring (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
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	$ 196		$ 208
Mortgage loans held for investment, at amortized cost	449,318		466,594
Total assets at fair value	478,138		495,561
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	124		135
Other assets	135		384
Total assets at fair value	27,224		29,450
Fannie Mae [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	223,862		232,170
Consolidated Trusts [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	12,602		10,800
Consolidated Trusts [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	225,456		234,424
Single-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	2,915		3,692
Single-Family [Member] | Fannie Mae [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	22,020	[1]	23,314	[1]
Single-Family [Member] | Consolidated Trusts [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	161	[1]	227	[1]
Multifamily [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	1,846		1,624
Acquired property, net	23		74
Proprietary home price model and appraisals [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Number of properties measured	73.00%		78.00%
Consensus without Inputs [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	124		135
Internal Model [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	94		203
Internal Model [Member] | Single-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	656		1,014
Internal Model [Member] | Single-Family [Member] | Fannie Mae [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	22,020		23,314
Internal Model [Member] | Single-Family [Member] | Consolidated Trusts [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	161		227
Appraisals [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	13		8
Appraisals [Member] | Single-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	853		467
Appraisals [Member] | Multifamily [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	186		194
Acquired property, net	1		8
Walk Forwards [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	5		43
Walk Forwards [Member] | Single-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	514		1,348
Broker Price Opinions [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	23
Broker Price Opinions [Member] | Multifamily [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	407		395
Acquired property, net	20		46
Asset Manager Estimate [Member] | Multifamily [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	1,231		1,001
Other Valuation Technique [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets			130
Other Valuation Technique [Member] | Single-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	71		76
Other Valuation Technique [Member] | Multifamily [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at amortized cost	22		34
Accepted Offers [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Number of properties measured	25.00%		20.00%
Accepted Offers [Member] | Single-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	821		787
Accepted Offers [Member] | Multifamily [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	$ 2		$ 20

[1] Excludes estimated recoveries from mortgage insurance proceeds.