Fair Value Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Trading Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 1,645	$ 2,669	$ 3,030	$ 2,774
Total gains or (losses) (realized/unrealized) Included in Net Income	(27)	122	39	496
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(1,067)	(264)
Issues	0	0	0	0
Settlements	(28)	(59)	(120)	(216)
Transfers out of Level 3	0	(16)	(322)	(200)
Transfers into Level 3	0	412	30	538
Ending Balance	1,590	3,128	1,590	3,128
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(27)	116	(22)	449
Trading Securities [Member] | Fannie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	305	42
Total gains or (losses) (realized/unrealized) Included in Net Income		0	(27)	(1)
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income		0	0	0
Purchases		0	0	0
Sales		0	(2)	0
Issues		0	0	0
Settlements		0	0	(2)
Transfers out of Level 3		0	(278)	(39)
Transfers into Level 3		325	2	325
Ending Balance	0	325	0	325
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end		0	0	0
Trading Securities [Member] | Freddie Mac [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance				2
Total gains or (losses) (realized/unrealized) Included in Net Income				0
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income				0
Purchases				0
Sales				0
Issues				0
Settlements				0
Transfers out of Level 3				(2)
Transfers into Level 3				0
Ending Balance		0		0
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end				0
Trading Securities [Member] | Alt-A private-label securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	325	643	597	618
Total gains or (losses) (realized/unrealized) Included in Net Income	(3)	2	41	105
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(267)	(23)
Issues	0	0	0	0
Settlements	(7)	(21)	(40)	(59)
Transfers out of Level 3	0	(16)	(44)	(159)
Transfers into Level 3	0	87	28	213
Ending Balance	315	695	315	695
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(3)	3	(3)	100
Trading Securities [Member] | Subprime private-label securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	718	1,282	1,307	1,448
Total gains or (losses) (realized/unrealized) Included in Net Income	(5)	66	38	245
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(580)	(241)
Issues	0	0	0	0
Settlements	(17)	(32)	(69)	(136)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	696	1,316	696	1,316
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(5)	66	(2)	209
Trading Securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	602	643	722	565
Total gains or (losses) (realized/unrealized) Included in Net Income	(19)	53	(17)	137
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(118)	0
Issues	0	0	0	0
Settlements	(4)	(3)	(8)	(9)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	579	693	579	693
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(19)	46	(17)	130
Trading Securities [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		101	99	99
Total gains or (losses) (realized/unrealized) Included in Net Income		1	4	10
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income		0	0	0
Purchases		0	0	0
Sales		0	(100)	0
Issues		0	0	0
Settlements		(3)	(3)	(10)
Transfers out of Level 3		0	0	0
Transfers into Level 3		0	0	0
Ending Balance	0	99	0	99
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end		1	0	10
Available-for-sale securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	10,953	16,810	15,080	19,012
Total gains or (losses) (realized/unrealized) Included in Net Income	112	109	676	505
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(177)	226	(467)	831
Purchases	0	0	421	0
Sales	(774)	(329)	(4,194)	(2,056)
Issues	0	0	0	0
Settlements	(587)	(850)	(1,774)	(2,603)
Transfers out of Level 3	(1)	(337)	(540)	(1,217)
Transfers into Level 3	26	268	350	1,425
Ending Balance	9,552	15,897	9,552	15,897
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Available-for-sale securities [Member] | Fannie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	129	2	0	7
Total gains or (losses) (realized/unrealized) Included in Net Income	0	0	0	0
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	421	0
Sales	(122)	0	(425)	0
Issues	0	0	0	0
Settlements	(8)	0	(8)	(1)
Transfers out of Level 3	0	(2)	0	(8)
Transfers into Level 3	1	0	12	2
Ending Balance	0	0	0	0
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Available-for-sale securities [Member] | Freddie Mac [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	4	9	6	8
Total gains or (losses) (realized/unrealized) Included in Net Income	0	0	0	0
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	0	(1)	(1)	(1)
Transfers out of Level 3	(1)	(2)	(2)	(2)
Transfers into Level 3	1	0	1	1
Ending Balance	4	6	4	6
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Available-for-sale securities [Member] | Alt-A private-label securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,654	3,717	3,140	3,791
Total gains or (losses) (realized/unrealized) Included in Net Income	2	15	174	131
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(8)	(60)	(124)	(11)
Purchases	0	0	0	0
Sales	0	0	(1,108)	(320)
Issues	0	0	0	0
Settlements	(178)	(111)	(387)	(310)
Transfers out of Level 3	0	(333)	(538)	(1,207)
Transfers into Level 3	24	268	337	1,422
Ending Balance	1,494	3,496	1,494	3,496
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Available-for-sale securities [Member] | Subprime private-label securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,837	5,705	5,240	7,068
Total gains or (losses) (realized/unrealized) Included in Net Income	33	87	478	388
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(45)	89	(277)	270
Purchases	0	0	0	0
Sales	0	(317)	(1,325)	(1,666)
Issues	0	0	0	0
Settlements	(148)	(178)	(439)	(674)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	3,677	5,386	3,677	5,386
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Available-for-sale securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,171	4,560	4,023	5,253
Total gains or (losses) (realized/unrealized) Included in Net Income	4	(4)	44	(31)
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(29)	200	(56)	480
Purchases	0	0	0	0
Sales	(8)	(12)	(324)	(70)
Issues	0	0	0	0
Settlements	(205)	(478)	(754)	(1,366)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	2,933	4,266	2,933	4,266
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Available-for-sale securities [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	2,158	2,817	2,671	2,885
Total gains or (losses) (realized/unrealized) Included in Net Income	73	11	(20)	17
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(95)	(3)	(10)	92
Purchases	0	0	0	0
Sales	(644)	0	(1,012)	0
Issues	0	0	0	0
Settlements	(48)	(82)	(185)	(251)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	1,444	2,743	1,444	2,743
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0	0	0
Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,595	2,531	1,833	2,704
Total gains or (losses) (realized/unrealized) Included in Net Income	9	116	47	243
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	3	5	34
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(97)	(93)	(273)	(259)
Transfers out of Level 3	(77)	(750)	(331)	(1,063)
Transfers into Level 3	70	64	219	212
Ending Balance	1,500	1,871	1,500	1,871
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(24)	(13)	(17)	49
Net derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	4	20	45	(40)
Total gains or (losses) (realized/unrealized) Included in Net Income	79	(25)	(20)	52
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(32)	(9)	26	(25)
Transfers out of Level 3	0	0	0	(1)
Transfers into Level 3	9	5	9	5
Ending Balance	60	(9)	60	(9)
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	28	(9)	23	14
Long-term debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(839)	(823)	(890)	(1,473)
Total (Losses) Gains (Realized/Unrealized) Included in Net Loss	(23)	(24)	(14)	(163)
Total Gains or (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	(64)	0	(64)	(751)
Settlements	18	15	43	70
Transfers out of Level 3	33	34	142	1,565
Transfers into Level 3	(65)	(26)	(157)	(72)
Ending Balance	(940)	(824)	(940)	(824)
Net Unrealized Gain (Loss) Included in Net Loss Related to Liabilities Still Held at period end	(23)	(25)	5	(115)
Long-term debt [Member] | Consolidated Trusts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(493)	(498)	(527)	(518)
Total (Losses) Gains (Realized/Unrealized) Included in Net Loss	0	(14)	(8)	(48)
Total Gains or (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	(64)	0	(64)	(1)
Settlements	18	16	43	51
Transfers out of Level 3	33	34	142	100
Transfers into Level 3	(65)	(26)	(157)	(72)
Ending Balance	(571)	(488)	(571)	(488)
Net Unrealized Gain (Loss) Included in Net Loss Related to Liabilities Still Held at period end	0	(14)	11	(45)
Long-term debt [Member] | Senior Floating [Member] | Fannie Mae [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(346)	(325)	(363)	(955)
Total (Losses) Gains (Realized/Unrealized) Included in Net Loss	(23)	(10)	(6)	(115)
Total Gains or (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	(750)
Settlements	0	(1)	0	19
Transfers out of Level 3	0	0	0	1,465
Transfers into Level 3	0	0	0	0
Ending Balance	(369)	(336)	(369)	(336)
Net Unrealized Gain (Loss) Included in Net Loss Related to Liabilities Still Held at period end	$ (23)	$ (11)	$ (6)	$ (70)