Equity Method Investments - Textuals (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	9 Months Ended
	Apr. 01, 2017	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Successor
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investments and financing receivables			$ 66.4	$ 82.1
Successor | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investment, ownership percentage			50.00%
Successor | Financing Receivable | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, cash received from equity Interest		$ 35.2	$ 81.1
Predecessor | Financing Receivable | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, cash received from equity Interest	$ 31.1