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Employee Benefits, Defined Benefit Plans Narrative (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2017 | Dec. 31, 2016
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs | $ 11.5 | $ 20.0
Union VEBA |  |
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs | 17.1 |
Maximum contribution threshold | 12.8 |
Accrued Veba contingent contribution - total | 11.5 |
Salaried VEBA |  |
VEBA Postretirement Medical Obligations |  |
Accrued annual contribution to VEBAs | 2.9 |
Maximum contribution threshold | 2.9 |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] |  |
VEBA Postretirement Medical Obligations |  |
Fair value of deferred compensation assets | $ 9.5 | $ 8.2

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