Commitments and Contingent Liabilities (Details) - USD ($) $ in Millions	6 Months Ended	66 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2014
Accrued Liabilities
Loss Contingencies [Line Items]
Accrual for black lung liability	$ 4.8	$ 4.8	$ 4.8
Uninsured Risk
Loss Contingencies [Line Items]
Accrual for black lung liability	50.8	50.8	$ 50.2
Haverhill and Granite City
Loss Contingencies [Line Items]
Estimate possible loss				$ 2.2
Actual spending for environmental remediation project		101.0
Environmental remediation expense	119.0
Haverhill and Granite City | Minimum
Loss Contingencies [Line Items]
Anticipated spending on environmental remediation project	140.0	140.0
Haverhill and Granite City | Maximum
Loss Contingencies [Line Items]
Anticipated spending on environmental remediation project	$ 145.0	$ 145.0