Supplemental Oil and Gas Disclosures (Results of Operations for Oil and Gas Producing Activities Disclosure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Sales	$ 2,525	$ 2,086	$ 1,413
Production (lifting) costs	(974)	(891)	(839)
Depreciation, depletion and amortization	(514)	(440)	(371)
Impairment of natural gas and oil properties			(2,321)
Results of operations - income before income taxes	1,037	755	(2,118)
Provision (benefit) for income taxes
Results of operations	1,037	755	(2,118)
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	$ 254	$ 287	$ 805