Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Treasury Stock	New Accounting Pronouncement, Early Adoption, Effect Accumulated Other Comprehensive Loss	New Accounting Pronouncement, Early Adoption, Effect Retained Earnings
Balance at beginning of period at Dec. 31, 2016				$ (76)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 700
Other comprehensive loss	21			21
Balance at end of period at Sep. 30, 2017				(55)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment of ASU adoption | Accounting Standards Update 2014-09					$ 208
Cumulative-effect adjustment of ASU adoption | Accounting Standards Update 2017-12							$ 3	$ (3)
Cumulative-effect adjustment of ASU adoption | Accounting Standards Update 2018-02							$ (3)	$ 3
Balance at beginning of period (in shares) at Dec. 31, 2017		791				376
Balance at beginning of period at Dec. 31, 2017	2,731	$ 8	$ 1,031	(54)	10,240	$ (8,494)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	901				901
Other comprehensive loss	(25)			(25)
Share-based compensation			54
Shares issued under stock plans			(40)			$ 58
Shares issued under stock plans (in shares)						(3)
Purchases of treasury stock (in shares)						17
Purchases of treasury stock						$ (1,226)
Balance at end of period (in shares) at Sep. 30, 2018		791				390
Balance at end of period at Sep. 30, 2018	$ 2,661	$ 8	$ 1,045	$ (79)	$ 11,349	$ (9,662)