Fair Value Level 3 Rollforward (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Trading Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 3,030	$ 2,774
Total (losses) or gains (realized/unrealized) Included in Net Income	15	139
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(410)	0
Issues	0	0
Settlements	(57)	(95)
Transfers out of Level 3	(322)	(117)
Transfers into Level 3	30	110
Ending Balance	2,286	2,811
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	127	138
Trading Securities [Member] | Fannie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	305	42
Total (losses) or gains (realized/unrealized) Included in Net Income	(27)	(1)
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	0	(2)
Transfers out of Level 3	(278)	(39)
Transfers into Level 3	2	0
Ending Balance	2	0
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	(19)	0
Trading Securities [Member] | Freddie Mac [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		2
Total (losses) or gains (realized/unrealized) Included in Net Income		0
Total (Losses) or gains (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
Net unrealized (losses) gains 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	597	618
Total (losses) or gains (realized/unrealized) Included in Net Income	8	19
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(17)	(16)
Transfers out of Level 3	(44)	(76)
Transfers into Level 3	28	110
Ending Balance	572	655
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	8	17
Trading Securities [Member] | Subprime private-label securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,307	1,448
Total (losses) or gains (realized/unrealized) Included in Net Income	1	76
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(398)	0
Issues	0	0
Settlements	(34)	(71)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	876	1,453
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	107	76
Trading Securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	722	565
Total (losses) or gains (realized/unrealized) Included in Net Income	35	40
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	(12)	0
Issues	0	0
Settlements	(3)	(4)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	742	601
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	33	40
Trading Securities [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	99	99
Total (losses) or gains (realized/unrealized) Included in Net Income	(2)	5
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(3)	(2)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	94	102
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	(2)	5
Available-for-sale securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	15,080	19,012
Total (losses) or gains (realized/unrealized) Included in Net Income	166	29
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	13	526
Purchases	239	0
Sales	(1,317)	(19)
Issues	0	0
Settlements	(712)	(864)
Transfers out of Level 3	(311)	(611)
Transfers into Level 3	313	390
Ending Balance	13,471	18,463
Net unrealized (losses) gains 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	7
Total (losses) or gains (realized/unrealized) Included in Net Income	0	0
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	239	0
Sales	(34)	0
Issues	0	0
Settlements	0	0
Transfers out of Level 3	0	(2)
Transfers into Level 3	0	0
Ending Balance	205	5
Net unrealized (losses) gains 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	6	8
Total (losses) or gains (realized/unrealized) Included in Net Income	0	0
Total (Losses) or gains (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	(1)	0
Transfers into Level 3	0	0
Ending Balance	5	8
Net unrealized (losses) gains 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	3,140	3,791
Total (losses) or gains (realized/unrealized) Included in Net Income	101	13
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	(78)	73
Purchases	0	0
Sales	(556)	0
Issues	0	0
Settlements	(124)	(88)
Transfers out of Level 3	(310)	(609)
Transfers into Level 3	313	390
Ending Balance	2,486	3,570
Net unrealized (losses) gains 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	5,240	7,068
Total (losses) or gains (realized/unrealized) Included in Net Income	180	33
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	(101)	219
Purchases	0	0
Sales	(565)	0
Issues	0	0
Settlements	(146)	(290)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	4,608	7,030
Net unrealized (losses) gains 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	4,023	5,253
Total (losses) or gains (realized/unrealized) Included in Net Income	23	(20)
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	52	193
Purchases	0	0
Sales	(162)	(19)
Issues	0	0
Settlements	(376)	(401)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	3,560	5,006
Net unrealized (losses) gains 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	2,671	2,885
Total (losses) or gains (realized/unrealized) Included in Net Income	(138)	3
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	140	41
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(66)	(85)
Transfers out of Level 3	0	0
Transfers into Level 3	0	0
Ending Balance	2,607	2,844
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	0	0
Mortgage Loans [Member] | Consolidated Trusts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,833	2,704
Total (losses) or gains (realized/unrealized) Included in Net Income	34	25
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	3	24
Sales	0	0
Issues	0	0
Settlements	(77)	(81)
Transfers out of Level 3	(57)	(148)
Transfers into Level 3	74	84
Ending Balance	1,810	2,608
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	19	11
Net derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	45	(40)
Total (losses) or gains (realized/unrealized) Included in Net Income	27	30
Total (Losses) or gains (realized/unrealized) Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	(6)	(12)
Transfers out of Level 3	0	(1)
Transfers into Level 3	0	0
Ending Balance	66	(23)
Net unrealized (losses) gains included in net income related to assets and liabilities still held at period end	39	11
Long-term debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(890)	(1,473)
Total (Losses) Gains (Realized/Unrealized) Included in Net Loss	(41)	(91)
Total Gains or (Losses) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	(750)
Settlements	9	38
Transfers out of Level 3	50	1,484
Transfers into Level 3	(66)	(24)
Ending Balance	(938)	(816)
Net Unrealized (Loss) Gain Included in Net Loss Related to Liabilities Still Held at period end	(40)	(45)
Long-term debt [Member] | Consolidated Trusts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(527)	(518)
Total (Losses) Gains (Realized/Unrealized) Included in Net Loss	(13)	(1)
Total Gains or (Losses) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	0
Settlements	9	18
Transfers out of Level 3	50	19
Transfers into Level 3	(66)	(24)
Ending Balance	(547)	(506)
Net Unrealized (Loss) Gain Included in Net Loss Related to Liabilities Still Held at period end	(12)	(1)
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	(363)	(955)
Total (Losses) Gains (Realized/Unrealized) Included in Net Loss	(28)	(90)
Total Gains or (Losses) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	0	0
Issues	0	(750)
Settlements	0	20
Transfers out of Level 3	0	1,465
Transfers into Level 3	0	0
Ending Balance	(391)	(310)
Net Unrealized (Loss) Gain Included in Net Loss Related to Liabilities Still Held at period end	$ (28)	$ (44)