Mortgage Loans - TDRs (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2021 USD ($) contract	Sep. 30, 2020 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average term extension of a single-family modified loan	151 months	159 months	146 months	166 months
Average interest rate reduction of a single-family modified loan	0.56%	0.42%	0.57%	0.35%
Number of loans, troubled debt restructurings activity | contract	2,441	7,052	9,420	29,667
Amortized cost, troubled debt restructurings activity | $	$ 371	$ 1,119	$ 1,401	$ 4,875
Number of loans, troubled debt restructurings subsequent default | contract	2,126	3,199	6,803	12,748
Amortized cost, troubled debt restructurings subsequent default | $	$ 342	$ 568	$ 1,125	$ 2,324
Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	2,441	7,052	9,420	29,667
Amortized cost, troubled debt restructurings activity | $	$ 371	$ 1,119	$ 1,401	$ 4,875
Number of loans, troubled debt restructurings subsequent default | contract	2,126	3,199	6,803	12,744
Amortized cost, troubled debt restructurings subsequent default | $	$ 342	$ 568	$ 1,125	$ 2,308
Multifamily
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	0	0	0	0
Amortized cost, troubled debt restructurings activity | $	$ 0	$ 0	$ 0	$ 0
Number of loans, troubled debt restructurings subsequent default | contract	0	0	0	4
Amortized cost, troubled debt restructurings subsequent default | $	$ 0	$ 0	$ 0	$ 16
20- and 30-year or more, amortizing fixed-rate | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	2,126	5,993	8,022	25,360
Amortized cost, troubled debt restructurings activity | $	$ 345	$ 1,006	$ 1,274	$ 4,412
Number of loans, troubled debt restructurings subsequent default | contract	1,782	2,956	5,754	11,529
Amortized cost, troubled debt restructurings subsequent default | $	$ 294	$ 532	$ 969	$ 2,119
15-year or less, amortizing fixed-rate | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	209	579	886	2,448
Amortized cost, troubled debt restructurings activity | $	$ 16	$ 50	$ 70	$ 216
Number of loans, troubled debt restructurings subsequent default | contract	133	21	330	106
Amortized cost, troubled debt restructurings subsequent default | $	$ 10	$ 1	$ 24	$ 7
Adjustable-rate | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	18	109	96	395
Amortized cost, troubled debt restructurings activity | $	$ 2	$ 16	$ 14	$ 62
Number of loans, troubled debt restructurings subsequent default | contract	11	3	26	15
Amortized cost, troubled debt restructurings subsequent default | $	$ 1	$ 0	$ 4	$ 2
Other | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	88	371	416	1,464
Amortized cost, troubled debt restructurings activity | $	$ 8	$ 47	$ 43	$ 185
Number of loans, troubled debt restructurings subsequent default | contract	200	219	693	1,094
Amortized cost, troubled debt restructurings subsequent default | $	$ 37	$ 35	$ 128	$ 180