Supplemental Quarterly Results (Schedule of Quarterly Financial Inform) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Quarterly Results [Abstract]
Operating revenues	$ 809	$ 737	$ 811	$ 846	$ 684	$ 651	$ 522	$ 579	$ 3,203	$ 2,436	$ 3,133
Operating income (loss)	167	110	188	266	122	(725)	(492)	(1,100)	731	(2,195)	(6,522)
Net income (loss) attributable to common stock	$ 267	$ 43	$ 224	$ 281	$ (237)	$ (735)	$ (620)	$ (1,159)	$ 815	$ (2,751)	$ (4,662)
Earnings (loss) per share - Basic	$ 0.53	$ 0.09	$ 0.45	$ 0.57	$ (0.48)	$ (1.52)	$ (1.61)	$ (3.03)	$ 1.64	$ (6.32)	$ (12.25)
Earnings (loss) per share - Diluted	$ 0.53	$ 0.09	$ 0.45	$ 0.57	$ (0.48)	$ (1.52)	$ (1.61)	$ (3.03)	$ 1.63	$ (6.32)	$ (12.25)
Impairment of natural gas and oil properties					$ 0	$ 817	$ 470	$ 1,034	$ 0	$ 2,321	$ 6,950
Participating securities - mandatory convertible preferred stock									$ 123