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Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
AOCI Attributable To Parent, Tax [Roll Forward] |  |  |  |  |  |  |
Beginning Balance, income taxes |  | $ (48) | $ 30 |  | $ (39) | $ 26 |
Changes in value, income taxes |  | 0 | 8 |  | (6) | 18 |
Amounts reclassified to earnings, income taxes |  | (4) | (6) |  | (7) | (12) |
Amounts reclassified to retained earnings, income taxes |  |  |  |  | 0 |  |
Net change, income taxes |  | (4) | 2 |  | (13) | 6 |
Ending Balance, income taxes |  | (52) | 32 |  | (52) | 32 |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |
Beginning Balance, net of tax | [1] |  |  |  | 8,734 |  |
Total other comprehensive income (loss), net |  | 12 | (8) | [2] | 45 | (15) | [2]
Ending Balance, net of tax |  | 8,639 |  |  | 8,639 |  |
Pension and Other Postretirement Liabilities |  |  |  |  |  |  |
AOCI Attributable To Parent, Before Tax [Roll Forward] |  |  |  |  |  |  |
Beginning Balance, before tax |  | (1,063) | (867) |  | (1,102) | (854) |
Changes in value, before tax |  | 1 | (7) |  | 24 | (33) |
Amounts reclassified to earnings, before tax |  | 14 | 14 |  | 30 | 27 |
Amounts reclassified to retained earnings, before tax |  |  |  |  | 0 |  |
Net change, before tax |  | 15 | 7 |  | 54 | (6) |
Ending Balance, before tax |  | (1,048) | (860) |  | (1,048) | (860) |
Investments and Other |  |  |  |  |  |  |
AOCI Attributable To Parent, Before Tax [Roll Forward] |  |  |  |  |  |  |
Beginning Balance, before tax |  | (3) | 1 |  | (6) | (1) |
Changes in value, before tax |  | 1 | (17) |  | (3) | (17) |
Amounts reclassified to earnings, before tax |  | 0 | 0 |  | 0 | 2 |
Amounts reclassified to retained earnings, before tax |  |  |  |  | 7 |  |
Net change, before tax |  | 1 | (17) |  | 4 | (15) |
Ending Balance, before tax |  | (2) | (16) |  | (2) | (16) |
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |
Beginning Balance, net of tax |  | (1,114) | (836) |  | (1,147) | (829) |
Changes in value, net of tax |  | 2 | (16) |  | 15 | (32) |
Amounts reclassified to earnings, net of tax |  | 10 | 8 |  | 23 | 17 |
Amounts reclassified to retained earnings, net of tax |  |  |  |  | 7 |  |
Total other comprehensive income (loss), net |  | 12 | (8) |  | 45 | (15) |
Ending Balance, net of tax |  | $ (1,102) | $ (844) |  | $ (1,102) | $ (844) |
[1] | Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.
[2] | Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.

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- Definition Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, attributable to parent. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners). + References No definition available. + Details
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- Definition Amount before tax and reclassification adjustments of other comprehensive income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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