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Benefit Plans (Narrative) (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Curtailment | $ 55 |  |  |
Assumptions used in calculating assumed health care cost trend rate |  | 4.00% |  |
Future capital call commitments for limited partnership investments |  | $ 384 |  |
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments |  | 71.00% | 68.00% |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent |  | 50.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  | 70.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  | 6.00% |  |
Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years |  | 5 years |  |
Benefit expense for the Company's savings plan |  | $ 76 | $ 75 | $ 71
Additional Contribution, Management Discretion |  |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  | 80.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  | 6.00% |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount, Percent |  | 2.00% |  |
First Year of Employment |  |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  | 35.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  | 6.00% |  |
Pension Plans, Defined Benefit |  |  |  |
Settlements |  | $ (22) | (88) |
Settlements |  | (22) | (86) |
Defined benefit plan, accumulated benefit obligation |  | 2,749 | $ 2,729 |
Future capital call commitments for limited partnership investments |  | $ 100 |  |
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments |  | 85.00% | 86.00% |
Limited partnership equity related hedge fund strategy plan assets |  | 62.00% |  |
Limited partnership hedge fund multi strategy plan assets |  | 32.00% |  |
Limited partnership hedge fund strategy distressed investments plan assets |  | 6.00% |  |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  | $ 18 |  |
Other Postretirement Benefits Plan |  |  |  |
Settlements |  | 0 | $ 0 |
Settlements |  | $ 0 | $ 0 |
Defined benefit plan, health care cost trend rate assumed for next fiscal year |  | 4.00% | 4.00% | 4.00%
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  | $ 2 |  |
Minimum |  |  |  |
Defined benefit plan, plan assets, target allocation, percentage |  | 40.00% |  |
Company Contribution of Eligible Compensation, Depending on Age, Percent |  | 3.00% |  |
Maximum |  |  |  |
Defined benefit plan, plan assets, target allocation, percentage |  | 60.00% |  |
Company Contribution of Eligible Compensation, Depending on Age, Percent |  | 5.00% |  |

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