Mortgage Loans - TDRs (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) contract	Sep. 30, 2021 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average term extension of a single-family modified loan	151 months	146 months
Average interest rate reduction of a single-family modified loan	0.56%	0.57%
Number of loans, troubled debt restructurings activity | contract	2,441	9,420
Amortized cost, troubled debt restructurings activity | $	$ 371	$ 1,401
Number of loans, troubled debt restructurings subsequent default | contract	2,126	6,803
Amortized cost, troubled debt restructurings subsequent default | $	$ 342	$ 1,125
Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	2,441	9,420
Amortized cost, troubled debt restructurings activity | $	$ 371	$ 1,401
Number of loans, troubled debt restructurings subsequent default | contract	2,126	6,803
Amortized cost, troubled debt restructurings subsequent default | $	$ 342	$ 1,125
Multifamily
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	0	0
Amortized cost, troubled debt restructurings activity | $	$ 0	$ 0
Number of loans, troubled debt restructurings subsequent default | contract	0	0
Amortized cost, troubled debt restructurings subsequent default | $	$ 0	$ 0
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	8,022
Amortized cost, troubled debt restructurings activity | $	$ 345	$ 1,274
Number of loans, troubled debt restructurings subsequent default | contract	1,782	5,754
Amortized cost, troubled debt restructurings subsequent default | $	$ 294	$ 969
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	886
Amortized cost, troubled debt restructurings activity | $	$ 16	$ 70
Number of loans, troubled debt restructurings subsequent default | contract	133	330
Amortized cost, troubled debt restructurings subsequent default | $	$ 10	$ 24
Adjustable-rate | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	18	96
Amortized cost, troubled debt restructurings activity | $	$ 2	$ 14
Number of loans, troubled debt restructurings subsequent default | contract	11	26
Amortized cost, troubled debt restructurings subsequent default | $	$ 1	$ 4
Other | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	88	416
Amortized cost, troubled debt restructurings activity | $	$ 8	$ 43
Number of loans, troubled debt restructurings subsequent default | contract	200	693
Amortized cost, troubled debt restructurings subsequent default | $	$ 37	$ 128