Revenues - Narrative (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Jan. 01, 2018
Capitalized Contract Cost [Line Items]
Remaining performance obligation	$ 430,000,000	$ 430,000,000
Remaining performance obligation, expected recognition period	3 years	3 years
Impairment losses		$ 0
Increase driven by payments received or due in advance of satisfying performance obligations		348,000,000	$ 334,000,000
Revenue recognized that was included in contract liability	$ 55,000,000	120,000,000
Other long-term assets	94,000,000	94,000,000	$ 65,000,000	$ 77,000,000
Sales Commissions
Capitalized Contract Cost [Line Items]
Commissions amortization	3,000,000	5,000,000
Deferred commissions	12,000,000	12,000,000
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Capitalized Contract Cost [Line Items]
Other long-term assets				12,000,000
Prepaid expenses and other current assets
Capitalized Contract Cost [Line Items]
Contract assets recorded in prepaids and other current assets	$ 5,000,000	$ 5,000,000		$ 7,000,000