Fair Value Level 3 Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Trading securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 3,030	$ 2,774	$ 2,286
Total gains or (losses) (realized/unrealized) Included in Net Income	8	512	469
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	6	0
Sales	(1,200)	(299)	(50)
Issues	0	0	0
Settlements	(148)	(277)	(304)
Transfers out of Level 3	(322)	(267)	(438)
Transfers into Level 3	30	581	811
Ending Balance	1,398	3,030	2,774
Net Unrealized Gains (Losses) included in net income related to assets still held	(8)	486	430
Trading securities [Member] | Fannie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	305	42	68
Total gains or (losses) (realized/unrealized) Included in Net Income	(27)	(27)	(9)
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	6	0
Sales	(2)	0	0
Issues	0	0	0
Settlements	0	(2)	(17)
Transfers out of Level 3	(278)	(39)	0
Transfers into Level 3	2	325	0
Ending Balance	0	305	42
Net Unrealized Gains (Losses) included in net income related to assets still held	0	(18)	(9)
Trading securities [Member] | Freddie Mac [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	2	2
Total gains or (losses) (realized/unrealized) Included in Net Income		0	0
Total gains or (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)	0
Transfers into Level 3		0	0
Ending Balance		0	2
Net Unrealized Gains (Losses) included in net income related to assets still held		0	0
Trading securities [Member] | Ginnie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	1
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			(1)
Transfers out of Level 3			(3)
Transfers into Level 3			3
Ending Balance			0
Net Unrealized Gains (Losses) included in net income related to assets still held			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	104
Total gains or (losses) (realized/unrealized) Included in Net Income	36	88	256
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	(267)	(58)	0
Issues	0	0	0
Settlements	(45)	(81)	(115)
Transfers out of Level 3	(44)	(226)	(435)
Transfers into Level 3	28	256	808
Ending Balance	305	597	618
Net Unrealized Gains (Losses) included in net income related to assets still held	15	97	223
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	1,319
Total gains or (losses) (realized/unrealized) Included in Net Income	36	270	328
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	(611)	(241)	(50)
Issues	0	0	0
Settlements	(88)	(170)	(149)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	644	1,307	1,448
Net Unrealized Gains (Losses) included in net income related to assets still held	10	234	322
Trading securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	722	565	675
Total gains or (losses) (realized/unrealized) Included in Net Income	(41)	168	(101)
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	(220)	0	0
Issues	0	0	0
Settlements	(12)	(11)	(9)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	449	722	565
Net Unrealized Gains (Losses) included in net income related to assets still held	(33)	160	(101)
Trading securities [Member] | Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	99	99	117
Total gains or (losses) (realized/unrealized) Included in Net Income	4	13	(5)
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	(100)	0	0
Issues	0	0	0
Settlements	(3)	(13)	(13)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	0	99	99
Net Unrealized Gains (Losses) included in net income related to assets still held	0	13	(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	25,034
Total gains or (losses) (realized/unrealized) Included in Net Income	839	606	302
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(661)	932	1,665
Purchases	421	0	359
Sales	(4,926)	(2,193)	(4,016)
Issues	0	0	0
Settlements	(2,132)	(3,337)	(4,642)
Transfers out of Level 3	(540)	(1,748)	(3,373)
Transfers into Level 3	350	1,808	3,683
Ending Balance	8,431	15,080	19,012
Net Unrealized Gains (Losses) included in net income related to assets still held	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	0	7	29
Total gains or (losses) (realized/unrealized) Included in Net Income	0	0	0
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	(1)
Purchases	421	0	0
Sales	(425)	0	0
Issues	0	0	0
Settlements	(8)	(1)	(7)
Transfers out of Level 3	0	(8)	(14)
Transfers into Level 3	12	2	0
Ending Balance	0	0	7
Net Unrealized Gains (Losses) included in net income related to assets still held	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	6	8	10
Total gains or (losses) (realized/unrealized) Included in Net Income	0	0	0
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	(1)
Purchases	0	0	0
Sales	0	0	0
Issues	0	0	0
Settlements	(1)	(1)	(2)
Transfers out of Level 3	(2)	(2)	(1)
Transfers into Level 3	1	1	2
Ending Balance	4	6	8
Net Unrealized Gains (Losses) included in net income related to assets still held	0	0	0
Available-for-sale securities [Member] | Ginnie Mae [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		0	0
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			(1)
Transfers into Level 3			1
Ending Balance			0
Net Unrealized Gains (Losses) included in net income related to assets still held			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	6,564
Total gains or (losses) (realized/unrealized) Included in Net Income	215	172	144
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(173)	(26)	464
Purchases	0	0	0
Sales	(1,504)	(393)	(2,664)
Issues	0	0	0
Settlements	(436)	(471)	(1,040)
Transfers out of Level 3	(538)	(1,738)	(3,357)
Transfers into Level 3	337	1,805	3,680
Ending Balance	1,041	3,140	3,791
Net Unrealized Gains (Losses) included in net income related to assets still held	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	5,240	7,068	7,447
Total gains or (losses) (realized/unrealized) Included in Net Income	599	447	120
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(382)	301	1,527
Purchases	0	0	359
Sales	(1,575)	(1,730)	(1,317)
Issues	0	0	0
Settlements	(601)	(846)	(1,068)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	3,281	5,240	7,068
Net Unrealized Gains (Losses) included in net income related to assets still held	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	4,023	5,253	7,837
Total gains or (losses) (realized/unrealized) Included in Net Income	51	(32)	25
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(104)	554	(449)
Purchases	0	0	0
Sales	(410)	(70)	(35)
Issues	0	0	0
Settlements	(859)	(1,682)	(2,125)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	2,701	4,023	5,253
Net Unrealized Gains (Losses) included in net income related to assets still held	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	2,671	2,885	3,147
Total gains or (losses) (realized/unrealized) Included in Net Income	(26)	19	13
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	(2)	103	125
Purchases	0	0	0
Sales	(1,012)	0	0
Issues	0	0	0
Settlements	(227)	(336)	(400)
Transfers out of Level 3	0	0	0
Transfers into Level 3	0	0	0
Ending Balance	1,404	2,671	2,885
Net Unrealized Gains (Losses) included in net income related to assets still held	0	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	2,634
Total gains or (losses) (realized/unrealized) Included in Net Income	57	260	282
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	5	36	346
Sales	0	0	(393)
Issues	0	0	0
Settlements	(350)	(344)	(459)
Transfers out of Level 3	(377)	(1,113)	(352)
Transfers into Level 3	309	290	646
Ending Balance	1,477	1,833	2,704
Net Unrealized Gains (Losses) included in net income related to assets still held	(20)	53	50
Net derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	45	(40)	14
Total gains or (losses) (realized/unrealized) Included in Net Income	(55)	103	(165)
Total gains or (losses) (realized/unrealized) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	(4)	0	0
Settlements	169	(21)	97
Transfers out of Level 3	(7)	(2)	16
Transfers into Level 3	9	5	(2)
Ending Balance	157	45	(40)
Net Unrealized Gains (Losses) included in net income related to assets still held	(2)	77	(51)
Long-term Debt [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(890)	(1,473)	(1,528)
Total gains (losses) (realized/unrealized) Included in Net Income	(15)	(195)	(174)
Total gains (losses) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	(66)	(751)	(695)
Settlements	57	81	580
Transfers out of Level 3	228	1,576	434
Transfers Into Level 3	(179)	(128)	(90)
Ending Balance	(865)	(890)	(1,473)
Net unrealized Gains (Losses) included in net income related to Liabilities still held	11	(146)	(4)
Long-term Debt [Member] | Consolidated Trusts [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(527)	(518)	(1,128)
Total gains (losses) (realized/unrealized) Included in Net Income	(9)	(53)	(250)
Total gains (losses) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	(66)	(1)	(21)
Settlements	57	62	537
Transfers out of Level 3	228	111	434
Transfers Into Level 3	(179)	(128)	(90)
Ending Balance	(496)	(527)	(518)
Net unrealized Gains (Losses) included in net income related to Liabilities still held	17	(49)	(80)
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)	(400)
Total gains (losses) (realized/unrealized) Included in Net Income	(6)	(142)	76
Total gains (losses) Included in Other Comprehensive Income	0	0	0
Purchases	0	0	0
Sales	0	0	0
Issues	0	(750)	(674)
Settlements	0	19	43
Transfers out of Level 3	0	1,465	0
Transfers Into Level 3	0	0	0
Ending Balance	(369)	(363)	(955)
Net unrealized Gains (Losses) included in net income related to Liabilities still held	$ (6)	$ (97)	$ 76