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Retirement Plans and Other Postretirement Benefits - Narrative (Details) | 6 Months Ended
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Jun. 30, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |
Amount of other postretirement benefit trust assets for union employee medical costs | $ 186,000,000
Pension Benefits |
Contributions |
Voluntary employer contributions to pension plan | 50,000,000
Minimum employer contributions for the next three years | 0
Maximum employer contributions for the next two years (up to) | 250,000,000
Other Benefits |
Contributions |
Estimated future employer contributions in next three years | 0
Retiree medical claim reimbursements | $ 48,000,000

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