Income taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Income taxes (Textual) [Abstract]
Increase in gross amount of unrecognized tax benefits for period	$ 75
Decrease in unrecognized tax benefits that is reasonably possible within the succeeding twelve months due to potential tax settlements	$ 330
Effective tax rate for the period excluding the impact of the tax credits associated with the Puerto Rico excise tax	18.50%	18.80%
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 2012	3.75%
Effective excise tax rate for 2013	2.75%
Effective excise tax rate for 2014	2.50%
Effective excise tax rate for 2015	2.25%
Effective excise tax rate for 2016	1.00%