Available-for-sale investments (Fair Values by Classification) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Consolidated Balance Sheets location
Cash and cash equivalents	$ 3,800	$ 3,241	$ 4,144	$ 3,731
Marketable securities	37,878	34,844
Other assets	2,119	1,625
Total available-for-sale investments	41,318	37,781
Available-for-sale investments [Member]
Consolidated Balance Sheets location
Cash and cash equivalents	3,291	2,783
Marketable securities	37,878	34,844
Other assets	149	154
Total available-for-sale investments	$ 41,318	$ 37,781