Mortgage Loans Individually Impaired Loans - 2 (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	$ 97,988	$ 117,507
Individually impaired loans with related allowance recorded: total interest income recognized	1,023	1,197
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	95	128
Individually impaired loans with no related allowance recorded: average recorded investment	18,545	19,140
Individually impaired loans with no related allowance recorded: total interest income recognized	270	319
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	31	31
Individually impaired loans: average recorded investment	116,533	136,647
Individually impaired loans: total interest income recognized	1,293	1,516
Individually impaired loans: interest income recognized on a cash basis	126	159
Table Footnote [Abstract]
Individually impaired loans: total interest income recognized	1,293	1,516
Single-family [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	97,741	117,249
Individually impaired loans with related allowance recorded: total interest income recognized	1,021	1,197
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	95	128
Individually impaired loans with no related allowance recorded: average recorded investment	18,192	18,809
Individually impaired loans with no related allowance recorded: total interest income recognized	268	317
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	31	31
Table Footnote [Abstract]
Troubled debt restructuring recorded investment	113,900	117,200
Single-family [Member] \| Primary [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	77,842	88,411
Individually impaired loans with related allowance recorded: total interest income recognized	817	911
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	80	107
Individually impaired loans with no related allowance recorded: average recorded investment	15,195	15,007
Individually impaired loans with no related allowance recorded: total interest income recognized	220	243
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	28	26
Single-family [Member] \| Government [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	272	276
Individually impaired loans with related allowance recorded: total interest income recognized	3	3
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	0	0
Individually impaired loans with no related allowance recorded: average recorded investment	56	60
Individually impaired loans with no related allowance recorded: total interest income recognized	1	0
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	0	0
Single-family [Member] \| Alt-A [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	14,687	20,708
Individually impaired loans with related allowance recorded: total interest income recognized	156	212
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	11	16
Individually impaired loans with no related allowance recorded: average recorded investment	2,293	2,842
Individually impaired loans with no related allowance recorded: total interest income recognized	39	58
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	2	4
Single-family [Member] \| Other [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	4,940	7,854
Individually impaired loans with related allowance recorded: total interest income recognized	45	71
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	4	5
Individually impaired loans with no related allowance recorded: average recorded investment	648	900
Individually impaired loans with no related allowance recorded: total interest income recognized	8	16
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	1	1
Multifamily [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	247	258
Individually impaired loans with related allowance recorded: total interest income recognized	2	0
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	0	0
Individually impaired loans with no related allowance recorded: average recorded investment	353	331
Individually impaired loans with no related allowance recorded: total interest income recognized	2	2
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	0	0
Table Footnote [Abstract]
Troubled debt restructuring recorded investment	$ 188	$ 187

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