Revenue from Contracts With Customers - Contract balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Contract Liabilities (Current)	$ 57		$ 57		$ 45	$ 45
Contract Liabilities (Noncurrent)	8		8		8
Increase (decrease) current contract liabilities			12
Increase (decrease) noncurrent contract liabilities					$ 8
Total increase (decrease) in contract liabilities			12
Cash received on contract liabilities			171
Revenue recognized included in contract liabilities current			159
Revenue recognized from opening balance of contract liabilities			45
Revenue recognized from obligations satisfied or partially satisfied in prior periods	$ 4	$ 3	$ 10	$ 8