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, 2017	$ 11,708	$ 0	$ 937	$ (3,325)	$ 7,379	$ 6,779	$ (62)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	65			158	(93)
Shares repurchased	(8)			(8)
Stock-based compensation	(49)				(49)
Net income	356					356
Other comprehensive income (loss)	4						4
Dividends paid (per share amount of $0.05)	(42)					(42)
Ending balance at Mar. 31, 2018	12,034	0	937	(3,175)	7,237	7,093	(58)
Beginning balance at Dec. 31, 2017	11,708	0	937	(3,325)	7,379	6,779	(62)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	452
Other comprehensive income (loss)	21
Ending balance at Jun. 30, 2018	12,132	0	937	(3,137)	7,227	7,146	(41)
Beginning balance at Mar. 31, 2018	12,034	0	937	(3,175)	7,237	7,093	(58)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued - stock-based compensation	25			40	(15)
Shares repurchased	(2)			(2)
Stock-based compensation	5				5
Net income	96					96
Other comprehensive income (loss)	17						17
Dividends paid (per share amount of $0.05)	(43)					(43)
Ending balance at Jun. 30, 2018	12,132	0	937	(3,137)	7,227	7,146	(41)
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 on Retained Earnings, Net of Tax	(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 income (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 income (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 income (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