Fair Value Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Trading securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 3	$ 2	$ 3	$ 13
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	84	1,148	1,201	1,127
Total gains (losses) (realized/unrealized) Included in Net Income	(5)	7	80	18
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	63	1	63
Sales	0	(21)	(1,060)	(21)
Issues	0	0	0	0
Settlements	0	(14)	(6)	(25)
Transfers out of Level 3	0	(21)	(137)	(22)
Transfers into Level 3	1	974	1	996
Ending Balance	80	2,136	80	2,136
Trading securities [Member] | Fannie Mae [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	3	0	3	2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	83	856	971	835
Total gains (losses) (realized/unrealized) Included in Net Income	(5)	1	166	4
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	63	1	63
Sales	0	0	(1,060)	0
Issues	0	0	0	0
Settlements	0	(2)	0	(5)
Transfers out of Level 3	0	(21)	0	(22)
Transfers into Level 3	1	974	1	996
Ending Balance	79	1,871	79	1,871
Trading securities [Member] | Other agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)			0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance			35
Total gains (losses) (realized/unrealized) Included in Net Income			(1)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)			0
Purchases			0
Sales			0
Issues			0
Settlements			(1)
Transfers out of Level 3			(33)
Transfers into Level 3			0
Ending Balance	0		0
Trading securities [Member] | Alt-A and subprime private-label securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		2	0	11
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		272	194	271
Total gains (losses) (realized/unrealized) Included in Net Income		3	(85)	11
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)		0	0	0
Purchases		0	0	0
Sales		0	0	0
Issues		0	0	0
Settlements		(11)	(5)	(18)
Transfers out of Level 3		0	(104)	0
Transfers into Level 3		0	0	0
Ending Balance	0	264	0	264
Trading securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1	20	1	21
Total gains (losses) (realized/unrealized) Included in Net Income	0	3	0	3
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	(21)	0	(21)
Issues	0	0	0	0
Settlements	0	(1)	0	(2)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	1	1	1	1
Available-for-sale securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,119	2,039	1,313	2,153
Total gains (losses) (realized/unrealized) Included in Net Income	(3)	34	15	36
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	24	(54)	(40)	(43)
Purchases	0	0	0	0
Sales	(7)	(312)	(18)	(325)
Issues	0	0	0	0
Settlements	(40)	(49)	(173)	(163)
Transfers out of Level 3	0	(21)	(4)	(51)
Transfers into Level 3	0	0	0	30
Ending Balance	1,093	1,637	1,093	1,637
Available-for-sale securities [Member] | Fannie Mae [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	202	232	208	230
Total gains (losses) (realized/unrealized) Included in Net Income	1	0	1	1
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	4	(3)	0	(2)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(3)	(2)	(5)	(6)
Transfers out of Level 3	0	(21)	0	(47)
Transfers into Level 3	0	0	0	30
Ending Balance	204	206	204	206
Available-for-sale securities [Member] | Other agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance				5
Total gains (losses) (realized/unrealized) Included in Net Income				0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)				0
Purchases				0
Sales				(1)
Issues				0
Settlements				0
Transfers out of Level 3				(4)
Transfers into Level 3				0
Ending Balance		0		0
Available-for-sale securities [Member] | Alt-A and subprime private-label securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	27	205	77	217
Total gains (losses) (realized/unrealized) Included in Net Income	0	0	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	(21)	(45)	(15)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(1)	(6)	(2)	(24)
Transfers out of Level 3	0	0	(4)	0
Transfers into Level 3	0	0	0	0
Ending Balance	26	178	26	178
Available-for-sale securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	539	1,185	671	1,272
Total gains (losses) (realized/unrealized) Included in Net Income	(11)	34	0	35
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	10	(11)	(3)	(12)
Purchases	0	0	0	0
Sales	(7)	(312)	(18)	(324)
Issues	0	0	0	0
Settlements	(25)	(23)	(144)	(98)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	506	873	506	873
Available-for-sale securities [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	351	417	357	429
Total gains (losses) (realized/unrealized) Included in Net Income	7	0	14	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	10	(19)	8	(14)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(11)	(18)	(22)	(35)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	357	380	357	380
Mortgage loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	8	19	16	16
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,102	1,149	1,116	1,197
Total gains (losses) (realized/unrealized) Included in Net Income	11	24	28	32
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(79)	(55)	(127)	(117)
Transfers out of Level 3	(51)	(21)	(87)	(67)
Transfers into Level 3	35	22	88	74
Ending Balance	1,018	1,119	1,018	1,119
Net derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(14)	9	(37)	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	133	113	134	44
Total gains (losses) (realized/unrealized) Included in Net Income	(28)	27	(86)	100
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	12	(16)	16	(24)
Transfers out of Level 3	0	0	53	5
Transfers into Level 3	0	0	0	(1)
Ending Balance	117	124	117	124
Long-term debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	2	(19)	21	(22)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(819)	(564)	(958)	(588)
Total gains (losses) (realized/unrealized) Included in Net Income	7	(20)	29	(22)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	1	(2)
Settlements	21	12	31	19
Transfers out of Level 3	177	22	331	88
Transfers into Level 3	(59)	(575)	(107)	(620)
Ending Balance	(673)	(1,125)	(673)	(1,125)
Long-term debt [Member] | Consolidated Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(1)	(5)	(1)	(4)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(462)	(214)	(582)	(241)
Total gains (losses) (realized/unrealized) Included in Net Income	4	(5)	7	(4)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	1	(2)
Settlements	21	12	31	19
Transfers out of Level 3	177	22	331	88
Transfers into Level 3	(59)	(575)	(107)	(620)
Ending Balance	(319)	(760)	(319)	(760)
Long-term debt [Member] | Senior floating [Member] | Fannie Mae [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	3	(14)	22	(18)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(357)	(350)	(376)	(347)
Total gains (losses) (realized/unrealized) Included in Net Income	3	(15)	22	(18)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	0	0	0	0
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	$ (354)	$ (365)	$ (354)	$ (365)