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Benefit Plans - Cash flows narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Pension benefits |  |
Components of net periodic cost |  |
Defined benefit pension plans, estimated employer contributions in next fiscal year | $ 133 |
Employer contribution | 131 | $ 131
Postretirement benefits |  |
Components of net periodic cost |  |
Employer contribution | 23 | 25
Participant contributions | $ 12 | $ 16

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