Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income	Treasury Stock	Previously reported Retained Earnings	Previously reported Accumulated Other Comprehensive (Loss) Income
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 at Dec. 31, 2016		$ 683	$ 2,173	9,359	(173)	$ (73)	$ 9,359	$ (173)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			(6)			12
Dividends to common stockholders				(762)
Net income	$ 676			676
Other comprehensive (loss) income	280				280
Total stockholder's equity at end of period at Sep. 30, 2017	12,169	683	2,167	9,273	107	(61)
Total stockholder's equity at beginning of period at Jun. 30, 2017					32
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	144
Other comprehensive (loss) income	75
Total stockholder's equity at end of period at Sep. 30, 2017	12,169	683	2,167	9,273	107	(61)
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 at Dec. 31, 2017	12,244	683	2,175	9,364	16	(60)	$ 9,414	$ 32
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			11			2
Dividends to common stockholders				(805)
Net income	897			897
Other comprehensive (loss) income	(773)				(773)
Total stockholder's equity at end of period at Sep. 30, 2018	11,510	683	2,186	9,456	(757)	(58)
Total stockholder's equity at beginning of period at Jun. 30, 2018					(605)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	336
Other comprehensive (loss) income	(152)
Total stockholder's equity at end of period at Sep. 30, 2018	$ 11,510	$ 683	$ 2,186	$ 9,456	$ (757)	$ (58)