Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Line Items]
Operating revenues	$ 687	$ 749	$ 764	$ 933	$ 962	$ 928	$ 1,035	$ 1,113	$ 3,133	$ 4,038	$ 3,371
Operating income (loss)	(2,561)	(2,842)	(1,284)	165	285	286	367	435	(6,522)	1,373	1,204
Net income (loss) attributable to common stock	$ (2,134)	$ (1,766)	$ (815)	$ 46	$ 312	$ 211	$ 207	$ 194	$ (4,662)	$ 924	$ 704
Earnings per share - Basic	$ (5.58)	$ (4.62)	$ (2.13)	$ 0.12	$ 0.89	$ 0.60	$ 0.59	$ 0.55	$ (12.25)	$ 2.63	$ 2.01
Earnings per share - Diluted	$ (5.58)	$ (4.62)	$ (2.13)	$ 0.12	$ 0.88	$ 0.60	$ 0.59	$ 0.55	$ (12.25)	$ 2.62	$ 2.00
Impairment of natural gas and oil properties	$ 2,600	$ 2,800	$ 1,500						$ 6,950
Series B Preferred Stock [Member]
Quarterly Financial Data [Line Items]
Mandatory convertible preferred participating securities				$ 7