Income taxes (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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)
Balance at beginning of year	$ 1,200	$ 975	$ 920
Additions based on tax positions related to the current year	335	300	283
Additions based on tax positions related to prior years	96	5	1
Reductions for tax positions of prior years	(192)	(50)	(8)
Settlements	(24)	(30)	(221)
Balance at end of year	$ 1,415	$ 1,200	$ 975