Fair Value (Fair Value on a Non-Recurring Basis) (Details) - Fair Value, Level 3 [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired mortgage loans held for portfolio	$ 49	$ 52
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired mortgage loans held for portfolio	$ 1	$ 2