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Short-term Borrowings and Other Debt Obligations Schedule of Borrowing Availability (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]      
Credit agreement, maximum capacity $ 1,263    
Revolving Credit Facility [Member]      
Line of Credit Facility [Line Items]      
Short-term debt 25 $ 25 $ 0
Committed Letters of Credit [Member]      
Line of Credit Facility [Line Items]      
Letters of Credit Outstanding, Amount 131    
Term Loan Borrowings before Debt Issuance Cost [Member]      
Line of Credit Facility [Line Items]      
Debt and Capital Lease Obligations $ 363