Revenue from Contracts With Customers - Contract balances (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Contract Liabilities (Current)	$ 65	$ 65	$ 45
Contract Liabilities (Noncurrent)	6	6
Increase (decrease) current		20
Increase (decrease) noncurrent		6
Increase (decrease) in contract liabilities		26
Cash received on contract liabilities		116
Revenue recognized included in contract liabilities current		90	$ 45
Revenue recognized from obligations satisfied or partially satisfied in prior periods	$ 1	$ 5
Maximum
Contract with Customer, Asset and Liability [Abstract]
Payment term		1 year