Investments (Schedule of Held-to-Maturity Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity investment securities		$ 287	$ 321
Less than 12 Months, Fair Value	$ 149	112
Less than 12 Months, Unrealized Losses	0	0
12 Months or More, Fair Value	0	0
12 Months or More, Unrealized Losses	0	0
Fair Value	149	112
Unrealized Losses	0	0
Corporate And Other Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 Months, Fair Value	100	42
Less than 12 Months, Unrealized Losses	0	0
12 Months or More, Fair Value	0	0
12 Months or More, Unrealized Losses	0	0
Fair Value	100	42
Unrealized Losses	0	0
Commercial Paper [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 Months, Fair Value	49	70
Less than 12 Months, Unrealized Losses	0	0
12 Months or More, Fair Value	0	0
12 Months or More, Unrealized Losses	0	0
Fair Value	49	70
Unrealized Losses	0	0
Short-term Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity investment securities	321	287
Fair Value	321	287
Short-term Investments [Member] | Corporate And Other Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity investment securities	146	101
Fair Value	146	101
Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity investment securities	175	186
Fair Value	$ 175	$ 186