Supplemental Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Line Items]
Operating revenues	$ 684	$ 651	$ 522	$ 579	$ 687	$ 749	$ 764	$ 933	$ 2,436	$ 3,133	$ 4,038
Operating income (loss)	122	(725)	(492)	(1,100)	(2,561)	(2,842)	(1,284)	165	(2,195)	(6,522)	1,373
Net income (loss) attributable to common stock	$ (237)	$ (735)	$ (620)	$ (1,159)	$ (2,134)	$ (1,766)	$ (815)	$ 46	$ (2,751)	$ (4,662)	$ 924
Earnings (Loss) per share - Basic	$ (0.48)	$ (1.52)	$ (1.61)	$ (3.03)	$ (5.58)	$ (4.62)	$ (2.13)	$ 0.12	$ (6.32)	$ (12.25)	$ 2.63
Earnings (Loss) per share - Diluted	$ (0.48)	$ (1.52)	$ (1.61)	$ (3.03)	$ (5.58)	$ (4.62)	$ (2.13)	$ 0.12	$ (6.32)	$ (12.25)	$ 2.62
Impairment of natural gas and oil properties	$ 0	$ 817	$ 470	$ 1,034	$ 2,576	$ 2,839	$ 1,535		$ 2,321	$ 6,950
Series B Preferred Stock [Member]
Quarterly Financial Data [Line Items]
Participating securities - mandatory convertible preferred stock								$ 7