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Benefit Plans - Fair Value of pension plan assets (Details) - Pension benefits - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Components of net periodic cost |  |  |  |
Total plan assets at fair value | $ 2,882 | $ 2,233 |  |
Total plan assets at fair value, allocation (as a percent) | 100.00% | 100.00% |  |
Investments measured using the Net Asset Value practical expedient | $ 3,598 | $ 3,525 |  |
Securities lending obligation | (216) | (158) |  |
Other net plan assets | 20 | 50 |  |
Fair value of plan assets | 6,284 | 5,650 | $ 5,353 |
Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | $ 418 | $ 360 |  |
Total plan assets at fair value, allocation (as a percent) | 14.50% | 16.10% |  |
Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | $ 2,425 | $ 1,863 |  |
Total plan assets at fair value, allocation (as a percent) | 84.10% | 83.40% |  |
Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | $ 39 | $ 10 |  |
Total plan assets at fair value, allocation (as a percent) | 1.40% | 0.50% |  |
Fair value of plan assets | $ 39 | $ 10 | 18 | $ 27
Equity securities |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 419 | 302 |  |
Equity securities | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 126 | 155 |  |
Equity securities | Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 264 | 147 |  |
Equity securities | Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 29 | 0 |  |
Fair value of plan assets | 29 | 0 | 1 | 1
U.S. government and agencies |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 594 | 315 |  |
U.S. government and agencies | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 174 | 30 |  |
U.S. government and agencies | Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 420 | 285 |  |
U.S. government and agencies | Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | 0 |  |
Corporate |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 1,553 | 1,319 |  |
Corporate | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | 0 |  |
Corporate | Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 1,543 | 1,309 |  |
Corporate | Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 10 | 10 |  |
Fair value of plan assets | 10 | 10 | $ 10 | $ 12
Short-term investments |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 294 | 265 |  |
Short-term investments | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 97 | 144 |  |
Short-term investments | Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 197 | 121 |  |
Short-term investments | Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | 0 |  |
Cash and cash equivalents |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 21 | 32 |  |
Cash and cash equivalents | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 21 | 32 |  |
Cash and cash equivalents | Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | 0 |  |
Cash and cash equivalents | Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | 0 |  |
Free Standing Derivative Assets |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 1 | 0 |  |
Free Standing Derivative Assets | Quoted prices in active markets for identical assets (Level 1) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | (1) |  |
Free Standing Derivative Assets | Significant other observable inputs (Level 2) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 1 | 1 |  |
Free Standing Derivative Assets | Significant unobservable inputs (Level 3) |  |  |  |
Components of net periodic cost |  |  |  |
Total plan assets at fair value | 0 | $ 0 |  |
EMA limited partnerships |  |  |  |
Components of net periodic cost |  |  |  |
Investments measured using the Net Asset Value practical expedient | 3,200 |  |  |
Limited partnership interests |  |  |  |
Components of net periodic cost |  |  |  |
Investments measured using the Net Asset Value practical expedient | $ 402 |  |  |

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- Definition The fair value of plan assets before net receivables and reductions for liabilities other than for benefits. + References No definition available. + Details
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- Definition This element represents the allocation of total plan assets at fair value. + References No definition available. + Details
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- Definition Represents the plan's obligation to return securities lending collateral received under a securities lending program. Due to its relatively short-term nature, the outstanding balance of the obligation approximates fair value. + References No definition available. + Details
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- Definition Represents other net assets of the plan, including interest and dividends receivable and net receivables related to settlements of investment transactions, such as purchases and sales. + References No definition available. + Details
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- Definition Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258 + Details
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