Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investments and financing receivables	$ 48.4		$ 45.9
Income (Loss) from Equity Method Investments	3.5	$ 22.0
Depreciation, depletion and amortization and asset retirement obligation expenses	172.5	169.6
Net interest expense	35.8	36.3
Income tax provision	18.8	10.1
Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Income (Loss) from Equity Method Investments	$ 3.8	22.2
Equity method investment, ownership percentage	50.00%
Depreciation, depletion and amortization and asset retirement obligation expenses	$ 3.6	3.9
Net interest expense	2.2	3.6
Income tax provision	1.7	5.1
Middlemount Mine | Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, cash received from equity Interest	$ 1.1	$ 35.8