Accrued Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Accrued benefits	$ 14.3	$ 21.4
Current portion of postretirement benefit obligation	3.3	3.3
Other taxes payable	12.7	10.4
Current portion of black lung liability	4.8	4.8
Accrued legal	3.6	4.4
Other	4.9	5.5
Total accrued liabilities	$ 43.6	$ 49.8