Investments (Contractual Maturities Of Investments In Debt Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Due within one year, available-for-sale	$ 299
Due within one year, held-to-maturity	826
Due after one year up to two years, available-for-sale	485
Due after one year up to two years, held-to-maturity	6
Due after two years up to three years, available-for-sale	179
Due after three years, available-for-sale	60
Asset-backed securities	341
Amortized Cost, available-for-sale	1,364
Amortized Cost, held-to-maturity	832	$ 1,071
Fair Value
Due within one year, available-for-sale	297
Due within one year, held-to-maturity	828
Due after one year up to two years, available-for-sale	482
Due after one year up to two years, available-for-sale	6
Due after two years up to three years, available-for-sale	180
Due after three years	61
Asset-backed securities	341
Fair Value, available-for-sale	1,361
Fair Value, held-to-maturity	$ 834	$ 1,070