Equity Method Investments and Financing Receivables (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Apr. 01, 2017	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investments and financing receivables	$ 75.0		$ 75.0	$ 75.0		$ 84.8
Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investment, ownership percentage	50.00%		50.00%	50.00%
Interest rate terms				15.00%
Financing Receivable | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of days available cash not required to pay known bills				35 days
Available cash distribution to stockholders ratio				50.00%
Available cash distribution to company if no marketing royalty payment overdue	100.00%
Financing receivable, cash received from equity Interest		$ 31.1	$ 35.2		$ 13.2
Intercompany loans						$ 50.0
Intercompany Loans, Outstanding	$ 0.0		$ 0.0	$ 0.0
Australian Bank Bill Swap Reference Rate | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Basis spread over Australian Bank Bill Swap Reference Rate	3.50%