Marketable Securities (Contractual maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Marketable securities by contractual maturity
Due in one year or less	$ 338
Due after one year through five years	13
Due after five years through ten years	14
Total marketable securities	$ 365	$ 1,103

Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year through fifth year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant