Mortgage Loans and Loan Loss Reserves - Detail of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Charge-offs	$ (47)		$ (220)
Allowance For Credit Loss
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	7,263	$ 4,854	7,263	$ 4,854
Single-family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance			4,222
Ending balance	7,137	4,841	7,137	4,841
Single-family | Allowance For Credit Loss
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	6,862	5,280	5,184	6,130
Provision (benefit) for credit losses	310	(181)	2,090	(480)
Charge-offs	(121)	(406)	(401)	(1,253)
Recoveries collected	41	107	165	341
Other	45	41	99	103
Ending balance	7,137	4,841	7,137	4,841
Single-family | Allowance for loan losses
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	6,647	4,841	6,647	4,841
Single-family | Allowance For Pre-Foreclosure Costs
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	383		383
Single-family | Allowance For Accrued Interest Receivable
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	107		107
Multifamily
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance			12
Multifamily | Allowance For Credit Loss
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	$ 126	$ 13	$ 126	$ 13