Investments (Fair Values by Classification) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair values of available-for-sale investments by classification in the Consolidated Balance Sheets
Cash and cash equivalents	$ 9,741	$ 3,800	$ 3,358	$ 3,241
Marketable securities	22,431	37,878
Other assets	1,982	2,119
Total available-for-sale investments	31,670	41,318
Available-for-sale investments [Member]
Fair values of available-for-sale investments by classification in the Consolidated Balance Sheets
Cash and cash equivalents	9,239	3,291
Marketable securities	22,431	37,878
Other assets	0	149
Total available-for-sale investments	$ 31,670	$ 41,318