Income taxes (Reconciliation of Total Gross Amounts of UTBs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of total gross amounts of unrecognized tax benefits (excluding interest, penalties, foreign tax credits and the federal tax benefit of state taxes related to unrecognized tax benefits)
Beginning balance	$ 2,543	$ 2,114	$ 1,772
Additions based on tax positions related to the current year	447	425	413
Additions based on tax positions related to prior years	1	18	9
Reductions for tax positions of prior years	(5)	(7)	(32)
Reductions for expiration of statute of limitations	(5)	0	0
Settlements	(28)	(7)	(48)
Ending balance	$ 2,953	$ 2,543	$ 2,114