Fair Value (Fair Value on a Non-Recurring Basis) (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired mortgage loans held for portfolio	$ 52	$ 57
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired mortgage loans held for portfolio	$ 2	$ 1