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Income Taxes (Deferred Tax Assets and Deferred Tax Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Deferred Tax Liabilities:    
Differences between book and tax basis of property $ (1,424) $ (1,926)
Other taxable temporary differences (143) (335)
Deferred Tax Liabilities, Gross 1,567 2,261
Deferred Tax Assets:    
Operating loss carryforwards 1,439 2,088
Capital loss carryforwards 63 59
Bad debt and other book provisions 66 96
Tax credit carryforwards 51 54
Other deductible temporary differences 60 263
Total gross deferred tax asset 1,679 2,560
Less: valuation allowance (988) (876)
Total net deferred tax asset 691 1,684
Total deferred tax liability $ (876) $ (577)