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 |