Share-based compensation (Pre-tax and After-tax Share-based Compensation Expense) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax		$ 268	$ 143	$ 128
Share-based compensation expense, after-tax		167	84	75
Stock options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax		33	31	28
Share-based compensation expense, after-tax	[1]	20	21	19
Additional Paid-in Capital
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Tax benefit on share-based compensation expense		$ 32	$ 26	$ 28

[1] (1) Excludes windfall tax benefit for share-based compensation recognized as a direct adjustment to Additional paid-in capital of $32 million, $26 million and $28 million for the years ended December 31, 2016, 2015 and 2014, respectively.