Supplemental Oil and Gas Disclosures (Results of Operations for Oil and Gas Producing Activities Disclosure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Sales	$ 2,086	$ 1,413	$ 2,074
Production (lifting) costs	(891)	(839)	(989)
Depreciation, depletion and amortization	(440)	(371)	(1,028)
Impairment of natural gas and oil properties		(2,321)	(6,950)
Results of operations - income before income taxes	755	(2,118)	(6,893)
Provision (benefit) for income taxes			(2,619)
Results of operations	755	(2,118)	$ (4,274)
Prior to Recognition of Valuation Allowance [Member]
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Provision (benefit) for income taxes	$ 287	$ 805