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Income Taxes (Tables) | 12 Months Ended
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Dec. 31, 2017
Income Tax Disclosure [Abstract] |
Components of Deferred Tax Assets and Liabilities | The components of deferred tax assets and liabilities at December 31, 2017 and 2016, are as follows:
(in millions) |  | 2017 |  | 2016
DEFERRED TAX LIABILITIES: |  |  |  |
Accelerated depreciation |  | $ | 3,193 |  |  | $ | 4,726 |
Other |  | 86 |  |  | 134 |
Total deferred tax liabilities |  | 3,279 |  |  | 4,860 |
DEFERRED TAX ASSETS: |  |  |  |
Fuel derivative instruments |  | 12 |  |  | 233 |
Construction obligation |  | 326 |  |  | 402 |
Accrued employee benefits |  | 309 |  |  | 451 |
Other |  | 274 |  |  | 400 |
Total deferred tax assets |  | 921 |  |  | 1,486 |
Net deferred tax liability |  | $ | 2,358 |  |  | $ | 3,374 |
Components of the Income Tax Provision | The provision for income taxes is composed of the following:
(in millions) |  | 2017 |  | 2016 |  | 2015
CURRENT: |  |  |  |  |  |
Federal |  | $ | 904 |  |  | $ | 778 |  |  | $ | 1,292 |
State |  | 72 |  |  | 69 |  |  | 114 |
Total current |  | 976 |  |  | 847 |  |  | 1,406 |
DEFERRED: |  |  |  |  |  |
Federal |  | 192 |  |  | 426 |  |  | (97 | )
State |  | 5 |  |  | 30 |  |  | (11 | )
Change in federal statutory tax rate |  | (1,410 | ) |  | — |  |  | — |
Total deferred |  | (1,213 | ) |  | 456 |  |  | (108 | )
|  | $ | (237 | ) |  | $ | 1,303 |  |  | $ | 1,298 |
Income Tax Provision Reconciliation To Federal Income Tax Statutory Rate | The effective tax rate on income before income taxes differed from the federal income tax statutory rate for the following reasons:
(in millions) |  | 2017 |  | 2016 |  | 2015
Tax at statutory U.S. tax rates |  | $ | 1,138 |  |  | $ | 1,241 |  |  | $ | 1,218 |
State income taxes, net of federal benefit |  | 50 |  |  | 64 |  |  | 66 |
Change in federal statutory tax rate |  | (1,410 | ) |  | — |  |  | — |
Other, net |  | (15 | ) |  | (2 | ) |  | 14 |
Total income tax provision |  | $ | (237 | ) |  | $ | 1,303 |  |  | $ | 1,298 |

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- Definition Tabular disclosure of the components of income tax expense attributable to continuing operations for each year presented including, but not limited to: current tax expense (benefit), deferred tax expense (benefit), investment tax credits, government grants, the benefits of operating loss carryforwards, tax expense that results from allocating certain tax benefits either directly to contributed capital or to reduce goodwill or other noncurrent intangible assets of an acquired entity, adjustments of a deferred tax liability or asset for enacted changes in tax laws or rates or a change in the tax status of the entity, and adjustments of the beginning-of-the-year balances of a valuation allowance because of a change in circumstances that causes a change in judgment about the realizability of the related deferred tax asset in future years. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319 + Details
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- Definition Tabular disclosure of the components of net deferred tax asset or liability recognized in an entity's statement of financial position, including the following: the total of all deferred tax liabilities, the total of all deferred tax assets, the total valuation allowance recognized for deferred tax assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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- Definition Tabular disclosure of the reconciliation using percentage or dollar amounts of the reported amount of income tax expense attributable to continuing operations for the year to the amount of income tax expense that would result from applying domestic federal statutory tax rates to pretax income from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 + Details
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