Share-based compensation (Pre-tax and After-tax Share-based Compensation Expense) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		$ 19	$ 48
Restricted stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax		235	259	$ 268
Share-based compensation expense, after-tax		178	151	167
Stock options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense, pre-tax		50	41	33
Share-based compensation expense, after-tax	[1]	40	26	20
Additional Paid-in Capital
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Tax benefit on share-based compensation expense		$ 0	$ 0	$ 32

[1] (1) Excludes windfall tax benefit for share-based compensation recognized as a direct adjustment to Additional paid-in capital of $32 million for the year ended December 31, 2016. Beginning in 2017, windfall tax benefits for share-based compensation are recognized through Net income rather than Additional paid-in capital. The excess tax benefit recorded to Income tax expense in the Consolidated statement of operations was $19 million and $48 million for the years ended December 31, 2018 and 2017, respectively.