Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 44.0		$ 44.0		$ 45.9
Income (Loss) from Equity Method Investments	9.7	$ 25.2	13.2	$ 47.2
Depreciation, depletion and amortization and asset retirement obligation expenses	165.4	163.9	337.9	333.5
Net interest expense	36.0	38.3	71.8	74.6
Income tax provision	3.0	7.4	21.8	17.5
Cash receipts from Middlemount Coal Pty			14.7	69.8
Middlemount Mine
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	$ 9.7	25.5	$ 13.5	47.7
Equity method investment, ownership percentage	50.00%		50.00%
Depreciation, depletion and amortization and asset retirement obligation expenses	$ 3.5	4.2	$ 7.1	8.1
Net interest expense	1.8	3.6	4.0	7.2
Income tax provision	$ 4.2	$ 6.4	5.9	11.5
Cash receipts from Middlemount Coal Pty			$ 14.7	$ 69.8