Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Taxes
Income tax expense	$ 56	$ 106	$ 139	$ 240
Effective tax rate (in percent)	21.50%	24.70%	21.80%	23.50%
Statutory income tax rate (in percent)	35.00%		35.00%