Investments (Trading Major Security Types) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 1,728	$ 888
US Treasury Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	873	0
U.S. obligations [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	140	150
US Government-sponsored Enterprises Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	64	60
Other Debt Obligations [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	262	259
Non-mortgage-backed securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	1,339	469
Multifamily [Member] | Mortgage-backed securities, GSE [Member]
Debt and Equity Securities, FV-NI [Line Items]
Trading securities	$ 389	$ 419