Short-term Borrowings and Other Debt Obligations Schedule of Borrowing Availability (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit agreement, maximum capacity	$ 1,238
Revolving Credit Facility
Line of Credit Facility [Line Items]
Short-term debt	0	$ 0
364 Day Secured RCF [Member]
Line of Credit Facility [Line Items]
Short-term debt	317
Committed Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	205
Term Loan Borrowings before Debt Issuance Cost [Member]
Line of Credit Facility [Line Items]
Debt and Capital Lease Obligations	$ 338