Acquisition of Shoal Creek Mine - Textuals (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 31, 2019	Dec. 03, 2018
Business Acquisition [Line Items]
Depreciation, depletion and amortization	$ 165.4			$ 163.9	$ 337.9	$ 333.5
Finite Lived Intangible Liabilities	71.1		$ 90.0		71.1
Revenues	1,149.0			1,309.4	2,399.6	2,772.1
Net income attributable to common stockholders	$ 37.1			$ 113.7	$ 161.3	220.3
Shoal Creek
Business Acquisition [Line Items]
Depreciation, depletion and amortization		$ 1.1	$ 0.2
Working Capital Adjustment							$ 2.4
Finite Lived Intangible Liabilities								$ 3.5
Business Acquisition, Pro Forma Taxable Income Impact						$ 0.0
Drummond Company, Inc.
Business Acquisition [Line Items]
Purchase price								$ 387.4