Mortgage Loans Individually Impaired Loans - 2 (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	$ 107,838	$ 123,183	$ 113,584	$ 128,873
Individually impaired loans with related allowance recorded: total interest income recognized	1,111	1,221	3,525	3,831
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	103	92	362	296
Individually impaired loans with no related allowance recorded: average recorded investment	18,878	19,758	18,988	19,592
Individually impaired loans with no related allowance recorded: total interest income recognized	330	365	971	1,123
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	36	30	105	84
Individually impaired loans: average recorded investment	126,716	142,941	132,572	148,465
Individually impaired loans: total interest income recognized	1,441	1,586	4,496	4,954
Individually impaired loans: interest income recognized on a cash basis	139	122	467	380
Table Footnote [Abstract]
Individually impaired loans: total interest income recognized	1,441	1,586	4,496	4,954
Single-family [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	107,607	122,951	113,340	128,602
Individually impaired loans with related allowance recorded: total interest income recognized	1,110	1,220	3,523	3,824
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	103	92	362	296
Individually impaired loans with no related allowance recorded: average recorded investment	18,543	19,454	18,655	19,298
Individually impaired loans with no related allowance recorded: total interest income recognized	322	359	960	1,107
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	36	30	105	84
Table Footnote [Abstract]
Troubled debt restructuring recorded investment	122,400		122,400	134,700
Single-family [Member] | Primary [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	84,043	90,941	86,842	94,594
Individually impaired loans with related allowance recorded: total interest income recognized	871	912	2,697	2,853
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	86	75	302	240
Individually impaired loans with no related allowance recorded: average recorded investment	15,140	15,402	15,039	15,173
Individually impaired loans with no related allowance recorded: total interest income recognized	254	273	740	835
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	30	24	88	71
Single-family [Member] | Government [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	276	289	277	295
Individually impaired loans with related allowance recorded: total interest income recognized	3	2	15	7
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	0	0	0	0
Individually impaired loans with no related allowance recorded: average recorded investment	57	61	58	61
Individually impaired loans with no related allowance recorded: total interest income recognized	1	0	3	2
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	0	0	0	0
Single-family [Member] | Alt-A [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	17,034	22,904	19,081	24,233
Individually impaired loans with related allowance recorded: total interest income recognized	179	228	610	717
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	13	13	45	42
Individually impaired loans with no related allowance recorded: average recorded investment	2,562	3,008	2,710	3,041
Individually impaired loans with no related allowance recorded: total interest income recognized	54	65	173	205
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	4	5	13	10
Single-family [Member] | Other [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	6,254	8,817	7,140	9,480
Individually impaired loans with related allowance recorded: total interest income recognized	57	78	201	247
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	4	4	15	14
Individually impaired loans with no related allowance recorded: average recorded investment	784	983	848	1,023
Individually impaired loans with no related allowance recorded: total interest income recognized	13	21	44	65
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	2	1	4	3
Multifamily [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	231	232	244	271
Individually impaired loans with related allowance recorded: total interest income recognized	1	1	2	7
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	0	0	0	0
Individually impaired loans with no related allowance recorded: average recorded investment	335	304	333	294
Individually impaired loans with no related allowance recorded: total interest income recognized	8	6	11	16
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	0	$ 0	0	0
Table Footnote [Abstract]
Troubled debt restructuring recorded investment	$ 210		$ 210	$ 185