Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Tax Disclosure [Abstract]
Effective income tax rate	15.20%	17.20%	15.50%	15.40%
Increase in gross amount of unrecognized tax benefits for period	$ 110		$ 220
Puerto Rico Excise Tax [Member]
Puerto Rico Excise Tax Rate [Line Items]
Excise tax rate effective through December 31, 2017			4.00%