Summary of Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Quarterly Financial Data [Abstract]
Revenues	$ 2,710	$ 3,064	$ 2,625	$ 2,862	$ 2,681	$ 2,910	$ 2,561	$ 2,776	$ 11,261	[1]	$ 10,928	[1]	$ 10,700	[1]
Other operating expenses	770	739	684	940	803	651	657	650	3,133		2,761		2,579
Pension and OPEB mark-to-market adjustment	(144)	0	0	0	(102)	0	0	0	(144)		(102)		(102)
Provision for depreciation	293	283	283	277	262	261	254	250	1,136		1,027		933
Impairment of assets and related charges	0	0	0	0	28	13	0	0	0		41		43
Operating Income	512	710	700	580	505	733	574	616	2,502		2,428		2,054
Income before income taxes	169	520	409	414	171	503	352	400
Income taxes	(13)	133	121	249	1,232	202	132	149	490		1,715		527
Income (loss) from continuing operations	182	387	288	165	(1,061)	301	220	251	1,022		(289)		551
Discontinued operations (Note 3)	(44)	(845)	11	1,204	(1,438)	95	(46)	(46)
Net Income (Loss)	138	(458)	299	1,369	(2,499)	396	174	205	1,348		(1,724)		(6,177)
Income allocated to preferred shareholders (2)	10	54	165	156	0	0	0	0
Net income (loss) attributable to common shareholders	$ 128	$ (512)	$ 134	$ 1,213	$ (2,499)	$ 396	$ 174	$ 205	$ 981		$ (1,724)		$ (6,177)
Earnings (loss) per share of common stock-
Basic - Continuing Operations, in dollars per share	$ 0.34	$ 0.66	$ 0.27	$ 0.01	$ (2.39)	$ 0.68	$ 0.49	$ 0.57	$ 1.33		$ (0.65)		$ 1.29
Basic - Discontinued Operations, in dollars per share	(0.09)	(1.68)	0.01	2.54	(3.23)	0.21	(0.10)	(0.11)	0.66		(3.23)		(15.78)
Basic - Net Income (Loss) Attributable to Common Stockholders, in dollars per share	0.25	(1.02)	0.28	2.55	(5.62)	0.89	0.39	0.46	1.99		(3.88)		(14.49)
Diluted - Continuing Operations, in dollars per share	0.34	0.66	0.27	0.01	(2.39)	0.68	0.49	0.57	1.33		(0.65)		1.29
Diluted - Discontinued Operations, in dollars per share	(0.09)	(1.68)	0.01	2.53	(3.23)	0.21	(0.10)	(0.11)	0.66		(3.23)		(15.78)
Diluted - Net Income (Loss) Attributable to Common Stockholders, in dollars per share	$ 0.25	$ (1.02)	$ 0.28	$ 2.54	$ (5.62)	$ 0.89	$ 0.39	$ 0.46	$ 1.99		$ (3.88)		$ (14.49)

[1] Includes excise and gross receipts tax collections of $386 million, $370 million and $378 million in 2018, 2017 and 2016, respectively.