Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Preferred Stock	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2018	$ 12,128	$ 0	$ 937	$ (3,816)	$ 7,238	$ 7,706	$ 63
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment	(31)					(31)
Shares issued - stock-based compensation	62			101	(39)
Shares repurchased	(30)			(30)
Stock-based compensation	(50)				(50)
Net income	174					174
Other comprehensive loss	(4)						(4)
Dividends paid (per share amount of $0.05)	(41)					(41)
Ending balance at Mar. 31, 2019	12,208	0	937	(3,745)	7,149	7,808	59
Beginning balance at Dec. 31, 2018	12,128	0	937	(3,816)	7,238	7,706	63
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	335
Other comprehensive loss	(13)
Ending balance at Jun. 30, 2019	12,101	0	937	(3,984)	7,170	7,928	50
Beginning balance at Mar. 31, 2019	12,208	0	937	(3,745)	7,149	7,808	59
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	2			(3)	5
Shares repurchased	(236)			(236)
Stock-based compensation	16				16
Net income	161					161
Other comprehensive loss	(9)						(9)
Dividends paid (per share amount of $0.05)	(41)					(41)
Ending balance at Jun. 30, 2019	12,101	0	937	(3,984)	7,170	7,928	50
Beginning balance at Dec. 31, 2019	12,153	0	937	(4,089)	7,207	7,993	105
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	38			121	(83)
Shares repurchased	(91)			(91)
Stock-based compensation	(22)				(22)
Net income	(46)					(46)
Other comprehensive loss	(22)						(22)
Dividends paid (per share amount of $0.05)	(40)					(40)
Ending balance at Mar. 31, 2020	11,958	0	937	(4,059)	7,102	7,895	83
Beginning balance at Dec. 31, 2019	12,153	0	937	(4,089)	7,207	7,993	105
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(796)
Other comprehensive loss	(68)
Ending balance at Jun. 30, 2020	11,175	0	937	(4,087)	7,143	7,145	37
Beginning balance at Mar. 31, 2020	11,958	0	937	(4,059)	7,102	7,895	83
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	(8)			(28)	20
Stock-based compensation	21				21
Net income	(750)					(750)
Other comprehensive loss	(46)						(46)
Ending balance at Jun. 30, 2020	$ 11,175	$ 0	$ 937	$ (4,087)	$ 7,143	$ 7,145	$ 37