Revenue Recognition - Practical Expedient (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Revenue Recognition
Practical expedient, disclosure of performance obligation	true
Contract with Customer, Asset and Liability
Revenue recognized included in deferred revenue at beginning of quarter	$ 68

Indicates (true/false) whether practical expedient is elected not to disclose amount of transaction price allocated to remaining performance obligation and explanation of expected timing of revenue recognition, when contract has expected duration of one year or less or right to consideration corresponds directly to performance completed.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130558-203045

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130557-203045

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84173487&loc=SL51747714-108306

Name:	us-gaap_RevenuePracticalExpedientRemainingPerformanceObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration