Mortgage Loans Credit Quality Indicators - SF (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	$ 3,189,975	$ 3,095,816
Single-Family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	2,922,974	2,863,935
Single-Family [Member] | Primary [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	2,802,795	2,715,497
Single-Family [Member] | Primary [Member] | Estimated mark-to-market loan-to-value ratio less than or equal to 80% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	2,439,858	2,321,201
Single-Family [Member] | Primary [Member] | Estimated mark-to-market loan-to-value ratio greater than 80% and less than or equal to 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	238,038	244,231
Single-Family [Member] | Primary [Member] | Estimated mark-to-market loan-to-value ratio greater than 90% and less than or equal to 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	106,076	114,412
Single-Family [Member] | Primary [Member] | Estimated mark-to-market loan-to-value ratio greater than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	18,823	35,653
Single-Family [Member] | Alt-A [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	67,226	81,947
Single-Family [Member] | Alt-A [Member] | Estimated mark-to-market loan-to-value ratio less than or equal to 80% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	51,903	56,250
Single-Family [Member] | Alt-A [Member] | Estimated mark-to-market loan-to-value ratio greater than 80% and less than or equal to 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	6,680	9,787
Single-Family [Member] | Alt-A [Member] | Estimated mark-to-market loan-to-value ratio greater than 90% and less than or equal to 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	4,044	6,731
Single-Family [Member] | Alt-A [Member] | Estimated mark-to-market loan-to-value ratio greater than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	4,599	9,179
Single-Family [Member] | Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	21,864	29,343
Single-Family [Member] | Other [Member] | Estimated mark-to-market loan-to-value ratio less than or equal to 80% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	16,428	19,382
Single-Family [Member] | Other [Member] | Estimated mark-to-market loan-to-value ratio greater than 80% and less than or equal to 90% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	2,277	3,657
Single-Family [Member] | Other [Member] | Estimated mark-to-market loan-to-value ratio greater than 90% and less than or equal to 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	1,443	2,627
Single-Family [Member] | Other [Member] | Estimated mark-to-market loan-to-value ratio greater than 100% [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	1,716	3,677
Single-Family [Member] | Government [Member]
Financing Receivable, Recorded Investment [Line Items]
Total recorded investment	$ 31,089	$ 37,148