Exit Activities (Details) - USD ($) $ in Millions	7 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	May. 31, 2013
Restructuring Cost and Reserve
Carlsbad restructuring expense	$ 0.0	$ 0.0	$ 125.4	$ 0.0
Hookers Prairie Mine Closing
Restructuring Cost and Reserve
Severance charges			11.0
Discontinued MOP Production at Carlsbad Facility
Restructuring Cost and Reserve
Carlsbad restructuring expense			125.4
Accelerated depreciation and depletion			100.0
Related tax benefits			$ 52.0