Equity Method Investments - Textuals (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Successor
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity method investments and financing receivables	$ 72.3
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	$ 35.8
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		$ 31.0