Fair Value Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Trading Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 1,398	$ 3,030
Total gains (losses) (realized/unrealized) Included in Net Income	(79)	15
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(282)	(410)
Issues	0	0
Settlements	(23)	(57)
Transfers out of Level 3	(363)	(322)
Transfers into Level 3	26	30
Ending Balance	677	2,286
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(57)	127
Trading Securities [Member] | Fannie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	305
Total gains (losses) (realized/unrealized) Included in Net Income	0	(27)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	0	(278)
Transfers into Level 3	25	2
Ending Balance	25	2
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	(19)
Trading Securities [Member] | Freddie Mac [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0
Total gains (losses) (realized/unrealized) Included in Net Income	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0
Purchases	0
Sales	0
Issues	0
Settlements	0
Transfers out of Level 3	0
Transfers into Level 3	1
Ending Balance	1
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	305	597
Total gains (losses) (realized/unrealized) Included in Net Income	(54)	8
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(6)	(17)
Transfers out of Level 3	0	(44)
Transfers into Level 3	0	28
Ending Balance	245	572
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(55)	8
Trading Securities [Member] | Subprime private-label securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	644	1,307
Total gains (losses) (realized/unrealized) Included in Net Income	(37)	1
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(187)	(398)
Issues	0	0
Settlements	(14)	(34)
Transfers out of Level 3	(363)	0
Transfers into Level 3	0	0
Ending Balance	43	876
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	(12)	107
Trading Securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	449	722
Total gains (losses) (realized/unrealized) Included in Net Income	12	35
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(95)	(12)
Issues	0	0
Settlements	(3)	(3)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	363	742
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	10	33
Trading Securities [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		99
Total gains (losses) (realized/unrealized) Included in Net Income		(2)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income		0
Purchases		0
Sales		0
Issues		0
Settlements		(3)
Transfers out of Level 3		0
Transfers into Level 3		0
Ending Balance		94
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end		(2)
Available-for-sale securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	8,431	15,080
Total gains (losses) (realized/unrealized) Included in Net Income	104	166
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	(163)	13
Purchases	0	239
Sales	(1,322)	(1,317)
Issues	0	0
Settlements	(305)	(712)
Transfers out of Level 3	(3,125)	(311)
Transfers into Level 3	1	313
Ending Balance	3,621	13,471
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	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	0	0
Total gains (losses) (realized/unrealized) Included in Net Income	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	239
Sales	0	(34)
Issues	0	0
Settlements	0	0
Transfers out of Level 3	0	0
Transfers into Level 3	1	0
Ending Balance	1	205
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	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	6
Total gains (losses) (realized/unrealized) Included in Net Income	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	(2)	(1)
Transfers into Level 3	0	0
Ending Balance	2	5
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	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	1,041	3,140
Total gains (losses) (realized/unrealized) Included in Net Income	12	101
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	(29)	(78)
Purchases	0	0
Sales	(291)	(556)
Issues	0	0
Settlements	(40)	(124)
Transfers out of Level 3	(516)	(310)
Transfers into Level 3	0	313
Ending Balance	177	2,486
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	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	3,281	5,240
Total gains (losses) (realized/unrealized) Included in Net Income	91	180
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	(139)	(101)
Purchases	0	0
Sales	(584)	(565)
Issues	0	0
Settlements	(104)	(146)
Transfers out of Level 3	(2,323)	0
Transfers into Level 3	0	0
Ending Balance	222	4,608
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	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	2,701	4,023
Total gains (losses) (realized/unrealized) Included in Net Income	4	23
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	30	52
Purchases	0	0
Sales	(43)	(162)
Issues	0	0
Settlements	(128)	(376)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	2,564	3,560
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0
Available-for-sale securities [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,404	2,671
Total gains (losses) (realized/unrealized) Included in Net Income	(3)	(138)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	(25)	140
Purchases	0	0
Sales	(404)	0
Issues	0	0
Settlements	(33)	(66)
Transfers out of Level 3	(284)	0
Transfers into Level 3	0	0
Ending Balance	655	2,607
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	0	0
Mortgage Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,477	1,833
Total gains (losses) (realized/unrealized) Included in Net Income	101	34
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	3
Sales	(320)	0
Issues	0	0
Settlements	(72)	(77)
Transfers out of Level 3	(65)	(57)
Transfers into Level 3	190	74
Ending Balance	1,311	1,810
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	13	19
Net derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	157	45
Total gains (losses) (realized/unrealized) Included in Net Income	180	27
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	(4)	0
Settlements	(100)	(6)
Transfers out of Level 3	(2)	0
Transfers into Level 3	0	0
Ending Balance	231	66
Net unrealized gains (losses) included in net income related to assets and liabilities still held at period end	66	39
Long-term debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(865)	(890)
Total gains (losses) (realized/unrealized) Included in Net Loss	(94)	(41)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	(7)	0
Settlements	309	9
Transfers out of Level 3	37	50
Transfers into Level 3	(21)	(66)
Ending Balance	(641)	(938)
Net unrealized gain (loss) included in net loss related to liabilities still held at period end	(28)	(40)
Long-term debt [Member] | Consolidated Trusts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(496)	(527)
Total gains (losses) (realized/unrealized) Included in Net Loss	(68)	(13)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	(7)	0
Settlements	309	9
Transfers out of Level 3	37	50
Transfers into Level 3	(21)	(66)
Ending Balance	(246)	(547)
Net unrealized gain (loss) included in net loss related to liabilities still held at period end	(2)	(12)
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	(369)	(363)
Total gains (losses) (realized/unrealized) Included in Net Loss	(26)	(28)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	(395)	(391)
Net unrealized gain (loss) included in net loss related to liabilities still held at period end	$ (26)	$ (28)