Available-for-sale Securities (Summary of Available-for-sale Securities in a Continuous Unrealized Loss Position) (Details) - Mortgage-backed Securities, Private-label residential [Member] $ in Millions	Jun. 30, 2019 USD ($) positions	Dec. 31, 2018 USD ($) positions
Debt Securities, Available-for-sale [Line Items]
Number of available-for-sale securities in unrealized loss position for less than 12 months | positions	1	1
Number of available-for-sale securities in unrealized loss position for 12 months or more | positions	2	2
Number of available-for-sale securities in unrealized loss position | positions	3	3
Estimated fair value of available-for-sale securities in unrealized loss position for less than 12 months	$ 3	$ 3
Estimated fair value of available-for-sale securities in unrealized loss position for 12 months or more	4	4
Estimated fair value of available-for-sale securities in unrealized loss position	7	7
Available For Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	(1)	(1)
Available For Sale Securities Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	$ (1)	$ (1)