Summary of Significant Accounting Policies Foreign Currency - Devaluation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impact of Foreign Currency Devaluation [Line Items]
Currency Devaluation Charges	$ 0	$ 41	$ 85
Angola, Kwanza
Impact of Foreign Currency Devaluation [Line Items]
Net Monetary Asset Position	$ 99