Income taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Puerto Rico Excise Tax Rate [Line Items]
Effective tax rate for the period excluding the impact of the tax credits associated with the new Pr excise tax	15.60%	18.50%	7.50%	18.60%
Income taxes (Textual) [Abstract]
Increase in gross amount of unrecognized tax benefits for period	$ 75		$ 155
Decrease in unrecognized tax benefits resulting from settlement with taxing authorities	10		200
Net tax benefit from settlement with taxing authorities	10		195
Decrease in unrecognized tax benefits that is reasonably possible within the succeeding 12 months due to resolution of federal and state audits	$ 70		$ 70
Puerto Rico Excise Tax [Member]
Puerto Rico Excise Tax Rate [Line Items]
Period that excise tax is imposed			6 years
Effective excise tax rate for 2011			4.00%
Effective excise tax rate for 2016			1.00%
Excise excise tax rate for July 2013 through December 31, 2017			4.00%