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Income Taxes (Deferred Tax Assets and Deferred Tax Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Deferred Tax Liabilities:    
Differences between book and tax basis of property $ (2,240) $ (2,364)
Other taxable temporary differences (299) (302)
Deferred Tax Liabilities, Gross 2,539 2,666
Deferred Tax Assets:    
Operating loss carryforwards 2,206 2,224
Capital loss carryforwards 66 137
Bad debt and other book provisions 191 221
Retirement costs 149 275
Tax credit carryforwards 55 58
Other deductible temporary differences 219 363
Total gross deferred tax asset 2,886 3,278
Less: valuation allowance (894) (997)
Total net deferred tax asset 1,992 2,281
Total deferred tax liability $ (547) $ (385)