Income Taxes (Components of Deferred Tax Balances) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities:
Differences between book and tax basis of property	$ 395	$ 81
Derivative activity	19
Other	1	1
Total deferred tax liabilities	415	82
Deferred tax assets
Accrued compensation	29	38
Alternative minimum tax credit carryforward		100
Accrued pension costs	14	19
Asset retirement obligations	41	53
Net operating loss carryforward	1,043	1,177
Derivative activity		142
Other	20	29
Total deferred tax assets	1,147	1,558
Valuation allowance	(732)	(1,476)
Net deferred tax liability