Income Taxes (Deferred Tax Assets and Deferred Tax Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Liabilities:
Differences between book and tax basis of property	$ (2,071)	$ (2,199)
Other taxable temporary differences	(80)	(328)
Deferred Tax Liabilities, Gross	2,151	2,527
Deferred Tax Assets:
Operating loss carryforwards	2,116	2,107
Capital loss carryforwards	59	66
Bad debt and other book provisions	182	156
Retirement costs	306	146
Tax credit carryforwards	54	55
Other deductible temporary differences	287	211
Total gross deferred tax asset	3,004	2,741
Less: valuation allowance	(876)	(894)
Total net deferred tax asset	2,128	1,847
Total deferred tax liability	$ (23)	$ (680)