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Postretirement Health Care and Life Insurance Benefits (Details 1) - Postretirement Health Care and Life Insurance Benefits [Member] $ in Millions | 12 Months Ended
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Dec. 31, 2016 USD ($) years | Dec. 31, 2015 USD ($) years | Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Percentage of actuarial gains and losses amortized |  | 0.00% |  |
Amortization period - future working lifetime of active employees | years |  | 10.31 | 10.49 |
Estimated net actuarial loss that will be amortized from accumulated other comprehensive (income) loss |  | $ 22.0 |  |
Prior service (cost) credit for the period |  | 9.2 |  |
Amounts recognized in accumulated other comprehensive loss |  |  |  |
Net actuarial loss (gain) arising during year |  | 32.3 | $ (35.1) | $ 115.8
Prior service credit arising during year |  | 0.0 | 0.0 | (18.0)
Amortization: |  |  |  |
Actuarial loss |  | (20.4) | (24.9) | (14.5)
Prior service credit (cost) |  | 9.2 | 6.8 | (1.3)
Settlement related to the Patriot bankruptcy |  |  |  |
Prior service cost | [1] | 7.2 | (16.6) | 0.0
Total recorded in other comprehensive (income) loss |  | $ 28.3 | $ (69.8) | $ 82.0
[1] | Refer to Note 27. "Matters Related to the Bankruptcy of Patriot Coal Corporation" herein for additional details related to this transaction.

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