Long-term and Short-term Debt (Short-term with Maturities of Year or Less) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Commercial paper	$ 100	$ 100
Other Short-term Borrowings	377	142
Short-term Debt	477	242
Average daily balance	$ 280	$ 135
Average days outstanding	27 days	21 days