Summary of Significant Accounting Policies New Accounting Pronouncements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ (388)	$ (304)	$ 715
Prepaid Taxes				$ 105
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income adjustment of non-service cost components	$ 41
New Accounting Pronouncement, Early Adoption, Effect [Member] | Accounting Standards Update 2016-09, Statutory Tax Withholding Component [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities		$ (10)	$ (9)