Fair Value Level 3 Valuation - Nonrecurring (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Consolidated Trusts [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	$ 10,800		$ 3,611
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	208
Mortgage loans held for investment	466,594
Total assets at fair value	495,561
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	232,170
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	234,424
Fair Value, Measurements, 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	135
Other assets	384
Total assets at fair value	29,450
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Proprietary home price model and appraisals [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Number of properties measured	78.00%
Fair Value, Measurements, 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	135
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Internal Model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	203
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Appraisals [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	8
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Walk Forwards [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	43
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other Valuation Technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Other assets	130
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Accepted Offers [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Number of properties measured	20.00%
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	3,692
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Internal Model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	1,014
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Appraisals [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	467
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Walk Forwards [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	1,348
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Other Valuation Technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	76
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Accepted Offers [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	787
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Fannie Mae [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	23,314	[1]
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Fannie Mae [Member] | Internal Model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	23,314
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Consolidated Trusts [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	227	[1]
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Single-Family [Member] | Consolidated Trusts [Member] | Internal Model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	227
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Multifamily [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	1,624
Acquired property, net	74
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Multifamily [Member] | Appraisals [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	194
Acquired property, net	8
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Multifamily [Member] | Broker Price Opinions [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	395
Acquired property, net	46
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Multifamily [Member] | Asset Manager Estimate [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	1,001
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Multifamily [Member] | Other Valuation Technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment	34
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Multifamily [Member] | Accepted Offers [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	$ 20

[1] Excludes estimated recoveries from mortgage insurance proceeds.