Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss)	Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax				$ 0	$ 0
Total stockholder's equity at beginning of period (Previously reported) at Dec. 31, 2016				9,359	(173)
Total stockholder's equity at beginning of period at Dec. 31, 2016		$ 683	$ 2,173	9,359	(173)	$ (73)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			(12)			8
Dividends paid to common stockholders				(613)
Net income	$ 260			260
Other comprehensive (loss) income	81				81
Total stockholder's equity at end of period at Mar. 31, 2017	11,693	683	2,161	9,006	(92)	(65)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax				(50)	(16)
Total stockholder's equity at beginning of period (Previously reported) at Dec. 31, 2017				9,414	32
Total stockholder's equity at beginning of period at Dec. 31, 2017	12,244	683	2,175	9,364	16	(60)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			(2)			1
Dividends paid to common stockholders				(627)
Net income	291			291
Other comprehensive (loss) income	(416)				(416)
Total stockholder's equity at end of period at Mar. 31, 2018	$ 11,425	$ 683	$ 2,173	$ 9,028	$ (400)	$ (59)