Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Adoption of Accounting Standard Update	Voting Common Stock	Paid-in Capital	Accumulated (Deficit) Income	Accumulated (Deficit) Income Adoption of Accounting Standard Update	Accumulated Other Comprehensive (Loss) Income, net of tax
Balance at Dec. 31, 2018	$ 659	$ (8)	$ 1	$ 1,788	$ (1,100)	$ (8)	$ (30)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201802Member
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	$ 8			8
Pension and OPEB liability adjustment	3						3
Available-for-sale securities and interest rate swaps	(21)						(21)
Foreign currency translation adjustment	1						1
Repurchase of common stock	(285)				(285)
Dividends on common stock	(37)				(37)
Net income	348				348
Balance at Jun. 30, 2019	668		1	1,796	(1,082)		(47)
Balance at Mar. 31, 2019	745		1	1,787	(1,010)		(33)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	5			5
Pension and OPEB liability adjustment	(3)						(3)
Available-for-sale securities and interest rate swaps	(12)						(12)
Foreign currency translation adjustment	1						1
Issuance of common stock	4			4
Repurchase of common stock	(235)				(235)
Dividends on common stock	(18)				(18)
Net income	181				181
Balance at Jun. 30, 2019	668		1	1,796	(1,082)		(47)
Balance at Dec. 31, 2019	781		1	1,802	(970)		(52)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	5			5
Pension and OPEB liability adjustment	(6)						(6)
Available-for-sale securities and interest rate swaps	(25)						(25)
Foreign currency translation adjustment	(2)						(2)
Issuance of common stock	(1)			(1)
Repurchase of common stock	(180)				(180)
Dividends on common stock	(39)				(39)
Net income	162				162
Balance at Jun. 30, 2020	695		1	1,806	(1,027)		(85)
Balance at Mar. 31, 2020	693		1	1,804	(1,031)		(81)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	2			2
Pension and OPEB liability adjustment	(2)						(2)
Available-for-sale securities and interest rate swaps	(2)						(2)
Dividends on common stock	(19)				(19)
Net income	23				23
Balance at Jun. 30, 2020	$ 695		$ 1	$ 1,806	$ (1,027)		$ (85)