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 (loss)	174					174
Other comprehensive income (loss)	(4)						(4)
Dividends paid	(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 (loss)	500
Other comprehensive income (loss)	49
Ending balance at Sep. 30, 2019	12,271	0	937	(4,028)	7,197	8,053	112
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 (loss)	161					161
Other comprehensive income (loss)	(9)						(9)
Dividends paid	(41)					(41)
Ending balance at Jun. 30, 2019	12,101	0	937	(3,984)	7,170	7,928	50
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	(5)			(14)	9
Shares repurchased	(30)			(30)
Stock-based compensation	18				18
Net income (loss)	165					165
Other comprehensive income (loss)	62						62
Dividends paid	(40)					(40)
Ending balance at Sep. 30, 2019	12,271	0	937	(4,028)	7,197	8,053	112
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 (loss)	(46)					(46)
Other comprehensive income (loss)	(22)						(22)
Dividends paid	(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 (loss)	(1,113)
Other comprehensive income (loss)	(48)
Ending balance at Sep. 30, 2020	10,892	0	937	(4,089)	7,159	6,828	57
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 (loss)	(750)					(750)
Other comprehensive income (loss)	(46)						(46)
Ending balance at Jun. 30, 2020	11,175	0	937	(4,087)	7,143	7,145	37
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	0			(2)	2
Stock-based compensation	14				14
Net income (loss)	(317)					(317)
Other comprehensive income (loss)	20						20
Ending balance at Sep. 30, 2020	$ 10,892	$ 0	$ 937	$ (4,089)	$ 7,159	$ 6,828	$ 57