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Other Comprehensive Income (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Other Comprehensive Income (Loss) before Reclassifications, Tax | $ (2) | $ 0 | $ 0
Income tax expense (benefit) | 181 | 542 | 581
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax | (2) | (2) | (5)
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Accumulated other comprehensive income (loss) at beginning of period | 11,455 |  |
Other comprehensive loss before reclassifications | (11) | (3) |
Amortization of net actuarial loss | 9 | 7 |
Net current period other comprehensive income (loss) | 1 | 7 |
Adoption of ASU No. 2018-02 |  | 0 |
Accumulated other comprehensive income (loss) at end of period | 12,222 | 11,455 |
Gains and Losses on Cash Flow Hedges |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Accumulated other comprehensive income (loss) at beginning of period | (58) | (51) |
Other comprehensive loss before reclassifications | (5) | 0 |
Amortization of net actuarial loss | 0 | 0 |
Net current period other comprehensive income (loss) | (2) | 3 |
Adoption of ASU No. 2018-02 |  | (10) |
Accumulated other comprehensive income (loss) at end of period | (60) | (58) | (51)
Defined Benefit Pension and Postretirement Items |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Reclassification from AOCI, Current Period, Tax | 3 | 5 |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Accumulated other comprehensive income (loss) at beginning of period | (67) | (59) |
Other comprehensive loss before reclassifications | (6) | (3) |
Amortization of net actuarial loss | 9 | 7 |
Net current period other comprehensive income (loss) | 3 | 4 |
Adoption of ASU No. 2018-02 |  | (12) |
Accumulated other comprehensive income (loss) at end of period | (64) | (67) | (59)
Total |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Accumulated other comprehensive income (loss) at beginning of period | (125) | (110) |
Adoption of ASU No. 2018-02 |  | (22) |
Accumulated other comprehensive income (loss) at end of period | (124) | (125) | $ (110)
Interest Rate Swap |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Amortization of net actuarial loss | 3 | 3 |
Interest Rate Swap | Gains and Losses on Cash Flow Hedges |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Amortization of net actuarial loss | 3 | 3 |
Interest Rate Swap | Defined Benefit Pension and Postretirement Items |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |
Amortization of net actuarial loss | 0 | 0 |
Reclassification out of Accumulated Other Comprehensive Income |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Income tax expense (benefit) | 0 | 0 |
Reclassification from AOCI, Current Period, Tax | 0 | 0 |
Reclassification out of Accumulated Other Comprehensive Income | Gains and Losses on Cash Flow Hedges |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Income tax expense (benefit) | $ 1 | $ 2 |

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