Share-based compensation (Pre-tax and After-tax Share-based Compensation Expense) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restricted stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax		$ 143	$ 128	$ 153
Share-based compensation expense, after-tax		84	75	89
Stock options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax		31	28	24
Share-based compensation expense, after-tax	[1]	21	19	18
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		$ 26	$ 28	$ 36

[1] (1) Excludes windfall tax benefit for share-based compensation recognized as a direct adjustment to Additional paid-in capital of $26 million, $28 million and $36 million for the years ended December 31, 2015, 2014 and 2013, respectively.