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Employee Benefit Plans and Other Postretirement Benefits - Additional Information (Details) - Qualified Plan - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Pension Contributions [Abstract] |  |  |  |
Minimum pension contributions |  |  | $ 4.6 |
Pension Plans |  |  |  |
Pension Contributions [Abstract] |  |  |  |
Contributions to pension plan | $ 18.0 | $ 36.2 | $ 21.2 | $ 42.1

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