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, 2018	12,128	0	937	(3,816)	7,238	7,706	63
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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)
Cumulative Effect on Retained Earnings, Net of Tax						(31)
Ending balance at Mar. 31, 2019	$ 12,208	$ 0	$ 937	$ (3,745)	$ 7,149	$ 7,808	$ 59