Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Voting Common Stock	Accumulated Other Comprehensive (Loss) Income, net of tax	Accumulated Other Comprehensive (Loss) Income, net of tax	Accumulated Other Comprehensive (Loss) Income, net of tax
Balance at Dec. 31, 2017	$ 689	$ 1	$ (15)	$ 1,758	$ (1,055)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	9			9
Pension and OPEB liability adjustment	(8)		(8)
Foreign currency translation adjustment	(5)		(5)
Available-for-sale securities and interest rate swaps	2		2
Issuance of common stock	17			17
Repurchase of common stock	(456)				(456)
Dividends on common stock	(61)				(61)
Impact of adopting accounting standards	5				5
Net income	492				492
Balance at Sep. 30, 2018	684	1	(26)	1,784	(1,075)
Balance at Jun. 30, 2018	602	1	(27)	1,764	(1,136)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	3			3
Pension and OPEB liability adjustment	(3)		(3)
Available-for-sale securities and interest rate swaps	4		4
Issuance of common stock	17			17
Repurchase of common stock	(86)				(86)
Dividends on common stock	(20)				(20)
Net income	167				167
Balance at Sep. 30, 2018	684	1	(26)	1,784	(1,075)
Balance at Dec. 31, 2018	659	1	(30)	1,788	(1,100)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	10			10
Pension and OPEB liability adjustment	1		1
Foreign currency translation adjustment	(5)		(5)
Available-for-sale securities and interest rate swaps	(26)		(26)
Issuance of common stock	1			1
Repurchase of common stock	(331)				(331)
Dividends on common stock	(55)				(55)
Impact of adopting accounting standards	(8)				(8)
Net income	497				497
Balance at Sep. 30, 2019	743	1	(60)	1,799	(997)
Balance at Jun. 30, 2019	668	1	(47)	1,796	(1,082)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	2			2
Pension and OPEB liability adjustment	(2)		(2)
Foreign currency translation adjustment	(6)		(6)
Available-for-sale securities and interest rate swaps	(5)		(5)
Issuance of common stock	1			1
Repurchase of common stock	(46)				(46)
Dividends on common stock	(18)				(18)
Net income	149				149
Balance at Sep. 30, 2019	$ 743	$ 1	$ (60)	$ 1,799	$ (997)