Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	8	12
Income (loss) from operations	$ 356	$ (50)
Discontinued operations	0	(4,907)
Net income (loss)	$ 356	$ (4,957)
Weighted average common shares outstanding, basic	851	849
Effect of dilutive securities	1	0
Weighted average common shares, diluted	852	849
Basic:
Income (loss) from continuing operations, per basic share	$ 0.42	$ (0.06)
Income (loss) from discontinued operations, per basic share	0.00	(5.78)
Net income (loss), per basic share	0.42	(5.84)
Diluted:
Income (loss) from continuing operations, per diluted share	0.42	(0.06)
Income (loss) from discontinued operations, per diluted share	0.00	(5.78)
Net income (loss), per diluted share	$ 0.42	$ (5.84)