Income Taxes (Deferred Tax Assets and Deferred Tax Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Liabilities:
Differences between book and tax basis of property	$ (1,418)	$ (1,424)
Other taxable temporary differences	(243)	(143)
Deferred Tax Liabilities, Gross	(1,661)	(1,567)
Deferred Tax Assets:
Operating loss carryforwards	1,066	1,439
Capital loss carryforwards	52	63
Bad debt and other book provisions	62	66
Tax credit carryforwards	55	51
Other deductible temporary differences	111	60
Total gross deferred tax asset	1,346	1,679
Less: valuation allowance	(868)	(988)
Total net deferred tax asset	478	691
Total deferred tax liability	$ (1,183)	$ (876)