Short-Term Debt and Credit Facilities Short-Term Debt (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term debt	[1]	$ 4,488	$ 1,869
Short-term Debt [Member] | Variable-rate Term Loan, 2.27%, expiring June 2018 [Member]
Short-term Debt [Line Items]
Short-term debt		$ 600
Debt, Weighted Average Interest Rate		2.27%

[1] (1)The table does not include unused credit facilities and letters of credit for investments that are accounted for under the equity method.