Supplemental Quarterly Financial Information (Narrative) (Details) - Revenues and Operating Income (Loss) [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017
Quarterly Financial Information, Quarterly Charges and Credits, Amount Reconciling to Previously Reported Results	$ 8
Immaterial Error Correction		Corporate interest income of $8 million previously reported in revenues and operating income (loss) in Q1 2017 has been reclassified to other (gains) losses, net.