Accrued Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Balance Sheet Related Disclosures [Abstract]
Accrued benefits	$ 17.1	$ 20.3
Other taxes payable	10.5	8.4
Accrued severance	3.0	4.7
Deferred revenue	11.3	2.1
Current portion of black lung liability	5.2	5.2
Other	8.3	4.3
Total accrued liabilities	$ 55.4	$ 45.0