Stockholders' equity (Share Repurchase Program) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Equity [Abstract]
Stock repurchased, Shares	3.9	4.7	3.3	2.9	8.6	6.2
Stock repurchased	$ 591	$ 690	$ 515	$ 451	$ 1,281	$ 966