Mortgage Loans - TDRs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average term extension of a single-family modified loan	138 months
Average interest rate reduction of a single-family modified loan	0.52%
Number of loans, troubled debt restructurings activity | contract	3,553
Amortized cost, troubled debt restructurings activity | $	$ 523
Number of loans, troubled debt restructurings subsequent default | contract	7,968
Amortized cost, troubled debt restructurings subsequent default | $	$ 1,452
Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	3,553
Amortized cost, troubled debt restructurings activity | $	$ 523
Number of loans, troubled debt restructurings subsequent default | contract	7,968
Amortized cost, troubled debt restructurings subsequent default | $	$ 1,452
Multifamily
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	0
Amortized cost, troubled debt restructurings activity | $	$ 0
Number of loans, troubled debt restructurings subsequent default | contract	0
Amortized cost, troubled debt restructurings subsequent default | $	$ 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	2,989
Amortized cost, troubled debt restructurings activity | $	$ 471
Number of loans, troubled debt restructurings subsequent default | contract	7,172
Amortized cost, troubled debt restructurings subsequent default | $	$ 1,327
15-year or less, amortizing fixed-rate | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	369
Amortized cost, troubled debt restructurings activity | $	$ 28
Number of loans, troubled debt restructurings subsequent default | contract	152
Amortized cost, troubled debt restructurings subsequent default | $	$ 11
Adjustable-rate | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	41
Amortized cost, troubled debt restructurings activity | $	$ 6
Number of loans, troubled debt restructurings subsequent default | contract	11
Amortized cost, troubled debt restructurings subsequent default | $	$ 2
Other | Single-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans, troubled debt restructurings activity | contract	154
Amortized cost, troubled debt restructurings activity | $	$ 18
Number of loans, troubled debt restructurings subsequent default | contract	633
Amortized cost, troubled debt restructurings subsequent default | $	$ 112