Accrued Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Balance Sheet Related Disclosures [Abstract]
Accrued benefits	$ 22.3	$ 20.3
Other taxes payable	12.6	8.4
Accrued severance	1.0	4.7
Current portion of black lung liability	5.2	5.2
Accrued legal	5.8	1.9
Other	3.1	2.4
Total accrued liabilities	$ 50.0	$ 42.9