Income taxes (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended		6 Months Ended
	Jun. 15, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2012	Jun. 30, 2012	Jun. 30, 2011
Income Taxes [Line Items]
Effective Income Tax Rate, Continuing Operations		32.20%	61.60%		34.10%	47.00%
Increase Decrease In Effective Tax Rate		(29.4)			(12.9)
Portion Of Charge Recorded As (Benefit) Provision For Income Taxes		$ (75)	$ 137		$ (75)	$ 137
Interest Benefit From Lower than Estimated Interest on Tax Underpayment		73
Cash Payment for Federal taxes and Interest	456
Expected Cash Payment for State Taxes and Interest				50
Interest On Tax Underpayments [Member]
Income Taxes [Line Items]
Portion Of Charge Recorded As (Benefit) Provision For Income Taxes			$ 312