Supplemental Cash Flow Statement Disclosures (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 603	$ 721	$ 477	$ 448
Current restricted cash	$ 6	$ 7	$ 8	$ 10
Location of current restricted cash	us-gaap:OtherAssetsCurrent	us-gaap:OtherAssetsCurrent	us-gaap:OtherAssetsCurrent	us-gaap:OtherAssetsCurrent
Noncurrent restricted cash			$ 1	$ 1
Location of noncurrent restricted cash	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Cash reported in assets held for sale	$ 63
Total cash, cash equivalents and restricted cash	672	$ 728	$ 486	$ 459
Other Non-cash items
PP&E acquired but not yet paid	83	$ 127
Cash settlement of treasury stock repurchases from prior period	$ 16