Mortgage Loans TDRs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) contracts	Dec. 31, 2019 USD ($) contracts	Dec. 31, 2018 USD ($) contracts
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average term extension of a single-family modified loan	163 months	162 months	109 months
Average interest rate reduction of a single-family modified loan	0.37%	0.13%	0.21%
Number of loans troubled debt restructurings activity | contracts	35,049	51,870	95,826
Amortized cost troubled debt restructurings activity | $	$ 5,665	$ 8,146	$ 14,427
Single-Family [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity | contracts	35,049	51,859	95,812
Amortized cost troubled debt restructurings activity | $	$ 5,665	$ 8,090	$ 14,353
Single-Family [Member] | Fixed Rate Residential Mortgage [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity | contracts	29,938	43,283	79,397
Amortized cost troubled debt restructurings activity | $	$ 5,125	$ 7,140	$ 12,485
Single-Family [Member] | FixedRateResidentialMortgage15yearMember [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity | contracts	2,956	4,762	8,953
Amortized cost troubled debt restructurings activity | $	$ 257	$ 424	$ 823
Single-Family [Member] | Adjustable Rate Residential Mortgage [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity | contracts	467	813	844
Amortized cost troubled debt restructurings activity | $	$ 72	$ 123	$ 129
Single-Family [Member] | Other [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity | contracts	1,688	3,001	6,618
Amortized cost troubled debt restructurings activity | $	$ 211	$ 403	$ 916
Multifamily [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity | contracts	0	11	14
Amortized cost troubled debt restructurings activity | $	$ 0	$ 56	$ 74