Share-based Compensation - Share-based Compensation Activity (Detail) - 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]
Share-based compensation expense before income taxes	$ 72	$ 86	$ 73
Income tax benefit on share-based compensation expense		1	28
Restricted Stock
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense before income taxes	40	56	45
Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense before income taxes	10	9	7
Performance Share Units
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense before income taxes	$ 22	$ 21	$ 21