Acquisition of Shoal Creek Mine Purchase Accounting (Details) - Shoal Creek - USD ($) $ in Millions	13 Months Ended
	Dec. 31, 2019	Dec. 03, 2018
Allocations
Inventories	$ 39.9	$ 39.7
Property, plant, equipment and mine development	365.3	364.7
Current liabilities	(4.1)	(4.1)
Asset retirement obligations	(11.3)	(10.5)
Total purchase price	389.8	$ 389.8
Adjustments
Fair Value Adjustment, Inventory	0.2
Fair Value Adjustment, Property, Plant, and Equipment	0.6
Fair Value Adjustment, Current liabilities	0.0
Fair Value Adjustment, Asset retirement obligations	0.8
Total purchase price	$ 0.0