Income Taxes (Components of Deferred Tax Balances) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities:
Differences between book and tax basis of property	$ 226	$ 395
Derivative activity	12	19
Other	2	1
Total deferred tax liabilities	240	415
Deferred tax assets
Accrued compensation	33	29
Accrued pension costs	10	14
Asset retirement obligations	15	41
Net operating loss carryforward	777	1,043
Other	14	20
Total deferred tax assets	849	1,147
Valuation allowance	(609)	(732)
Net deferred tax liability