Equity Method Investments - Textuals (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018	Dec. 31, 2017
Successor
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investments and financing receivables	$ 62.5			$ 62.5
Successor | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investment, ownership percentage	50.00%			50.00%
Successor | Financing Receivable | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, cash received from equity Interest	$ 34.1	$ 21.0		$ 69.8
Predecessor
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investments and financing receivables					$ 83.8
Predecessor | Financing Receivable | Middlemount Mine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, cash received from equity Interest			$ 31.1