Mortgage Loans Held for Portfolio (Mortgage Loans Held for Portfolio by Collateral or Guarantee Type) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable, Unpaid Principal Balance	$ 9,208	$ 7,734
Conventional Mortgage Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable, Unpaid Principal Balance	8,712	7,231
Loans Insured or Guaranteed by US Government Authorities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans And Leases Receivable, Unpaid Principal Balance	$ 496	$ 503