Share-based compensation (Pre-tax and After-tax Share-based Compensation Expense) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Restricted stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax	$ 128		$ 153		$ 109
Share-based compensation expense, after-tax	75		89		64
Stock options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax	28		24		22
Share-based compensation expense, after-tax	19	[1]	18	[1]	17	[1]
Additional Paid-in Capital
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	$ 28		$ 36		$ 18

[1] (1) Excludes windfall tax benefit for share-based compensation recognized as a direct adjustment to Additional paid-in capital of $28 million, $36 million and $18 million for the years ended December 31, 2014, 2013 and 2012, respectively.