Accrued Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Accrued liabilities
Accrued benefits	$ 20.3	$ 27.4
Other taxes payable	8.4	11.7
Accrued severance	4.7	13.0
Deferred revenue	2.1	0.0
Current portion of black lung liability	5.2	3.8
Other	5.1	15.4
Total accrued liabilities	$ 45.8	$ 71.3