Debt - Senior Notes and Convertible Senior Notes (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Long-Term Debt Instrument [Line Items]
Debt, principal	$ 8,168	$ 9,981
Minimum [Member] \| Convertible debt [Member]
Long-Term Debt Instrument [Line Items]
Debt, principal	50
Maximum [Member] \| Convertible debt [Member]
Long-Term Debt Instrument [Line Items]
Debt, principal	$ 75

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant