Mortgage Loans Held for Portfolio (Collateral or Guarantee Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed-rate single-family mortgages	$ 4,997,089	$ 5,030,333
Government-guaranteed/insured loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed-rate single-family mortgages	165,190	173,790
Conventional MPF Loan [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Fixed-rate single-family mortgages	$ 4,831,899	$ 4,856,543