Fair Value Measurements (Schedule of Cash and Cash Equivalents and Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	$ 1,001	$ 896	$ 663	$ 1,118
Short-term investments	722	1,213
Long-term investments	93	66
Total cash and cash equivalents and investments	1,816	2,175
Cash
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	931	846
Short-term investments	0	0
Long-term investments	0	0
Total cash and cash equivalents and investments	931	846
Commercial Paper
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	20	0
Short-term investments	125	124
Long-term investments	0	0
Total cash and cash equivalents and investments	145	124
Corporate Bonds
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	0	0
Short-term investments	398	642
Long-term investments	93	66
Total cash and cash equivalents and investments	491	708
Time Deposits
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	50	50
Short-term investments	199	447
Long-term investments	0	0
Total cash and cash equivalents and investments	$ 249	$ 497