Investments (Narrative) (Details) $ in Millions	Mar. 31, 2018 USD ($) positions	Dec. 31, 2017 USD ($)
Investment [Line Items]
Equity Securities	$ 1,947	$ 1,992
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | positions	18,000
Total number of security positions	28,000
Equity Method Investments	$ 959	$ 898