Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Line Items]
Income Tax Expense (Benefit), Intraperiod Tax Allocation	$ 1	$ 6	$ 0
NOL and credit carryforwards	1,595	1,810
Taxes related to risk management activities and derivatives	7	20
Reorganization items and impairments	166	146
Deferred Tax Assets, Other	101	28
Deferred tax assets before valuation allowance	1,869	2,004
Valuation allowance	(1,000)	(1,168)
Valuation Allowance, Deferred Tax Asset, Change in Amount	168	413	$ 56
Total deferred tax assets	869	836
Deferred tax liabilities: property, plant and equipment	(890)	(805)
Deferred Tax Liabilities, Gross	(890)	(805)
Deferred Tax Liabilities, Net	(21)
Net deferred tax asset (liability)		31
Deferred Tax Liabilities, Gross, Noncurrent	(22)	(28)
Deferred income tax asset, non-current	1	$ 59
Change in Valuation due to Merger [Member]
Valuation Allowance [Line Items]
Valuation Allowance, Deferred Tax Asset, Change in Amount	$ (58)