Mortgage Loans Held for Portfolio (Payment Status) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 9,547	$ 9,335
Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,787
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	6,253
Loans and Leases Receivable, Net of Deferred Income	9,040
Financing Receivable, Past Due		91
Financing Receivable, Not Past Due		8,783
Financing Receivable, before Allowance for Credit Loss		8,874
Conventional Mortgage Loan [Member] | Nonperforming Financial Instruments [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	61
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	22
Loans and Leases Receivable, Net of Deferred Income	83
Conventional Mortgage Loan [Member] | Performing Financial Instruments [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2,726
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	6,231
Loans and Leases Receivable, Net of Deferred Income	8,957
Financial Asset, 30 to 59 Days Past Due [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	35
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	16
Loans and Leases Receivable, Net of Deferred Income	51
Financing Receivable, Past Due		57
Financial Asset, 60 to 89 Days Past Due [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	12
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	2
Loans and Leases Receivable, Net of Deferred Income	14
Financing Receivable, Past Due		14
Financing Receivables, 90 to 179 Days Past Due [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	6
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	3
Loans and Leases Receivable, Net of Deferred Income	9
Financing Receivable, Past Due		10
Financing Receivables, Greater than 180 Days Past Due [Member] | Conventional Mortgage Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	8
Financing Receivable, Originated, Current Fiscal Year and Preceeding Four Preceeding Fiscal Years	1
Loans and Leases Receivable, Net of Deferred Income	$ 9
Financing Receivable, Past Due		$ 10