Fair Value Nonrecurring Changes (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	$ 0	$ 0
Mortgage loans held for investment	0	0
Total assets at fair value	57,431	52,939
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	169	185
Mortgage loans held for investment	2,687,759	2,599,667
Total assets at fair value	2,771,628	2,694,859
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	169	195
Mortgage loans held for investment	400,327	392,355
Total assets at fair value	419,874	415,630
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	0	0
Other assets	0	0
Total assets at fair value	0	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	93	101
Other assets	0	0
Total assets at fair value	93	101
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	110	132
Other assets	45	121
Total assets at fair value	22,416	25,970
Fannie Mae [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Fannie Mae [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	29,896	29,920
Fannie Mae [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	217,064	215,960
Consolidated Trusts [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	15,629	14,268
Consolidated Trusts [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Consolidated Trusts [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	2,657,863	2,569,747
Consolidated Trusts [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	183,263	176,395
Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	0	0
Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	0	0
Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	16,714	20,045
Acquired property, net	4,782	4,041
Single-Family [Member] | Fannie Mae [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Single-Family [Member] | Fannie Mae [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Single-Family [Member] | Fannie Mae [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	16,654	19,966
Single-Family [Member] | Consolidated Trusts [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Single-Family [Member] | Consolidated Trusts [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Single-Family [Member] | Consolidated Trusts [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	60	79
Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Acquired property, net	0	0
Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	0
Acquired property, net	0	0
Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	625	1,533
Acquired property, net	140	98
Consensus Without Inputs [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	110	132
Other Valuation Technique [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Other assets	45	121
Other Valuation Technique [Member] | Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	191	118
Other Valuation Technique [Member] | Single-Family [Member] | Consolidated Trusts [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	60	79
Other Valuation Technique [Member] | Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	0	55
Acquired property, net	13	89
Accepted Offers [Member] | Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	864	691
Internal Model [Member] | Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	1,045	1,049
Internal Model [Member] | Single-Family [Member] | Fannie Mae [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	16,654	19,966
Appraisals [Member] | Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	1,509	1,077
Broker Price Opinions [Member] | Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	45	248
Acquired property, net	127	9
Walk Forwards [Member] | Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	1,173	1,106
Asset Manager Estimate [Member] | Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	580	1,230
Estimated Fair Value [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	338	380
Mortgage loans held for investment	3,088,086	2,992,022
Total assets at fair value	3,243,747	3,155,006
Estimated Fair Value [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	203	233
Other assets	45	121
Total assets at fair value	22,509	26,071
Estimated Fair Value [Member] | Fannie Mae [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	246,960	245,880
Estimated Fair Value [Member] | Consolidated Trusts [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	2,841,126	2,746,142
Estimated Fair Value [Member] | Single-Family [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Acquired property, net	4,782	4,041
Estimated Fair Value [Member] | Single-Family [Member] | Fannie Mae [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	16,654	19,966
Estimated Fair Value [Member] | Single-Family [Member] | Consolidated Trusts [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	60	79
Estimated Fair Value [Member] | Multifamily [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets Measured on Nonrecurring Basis [Line Items]
Mortgage loans held for investment	625	1,533
Acquired property, net	$ 140	$ 98