Other Events (Details) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 AUD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Feb. 06, 2018
Successor
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable		$ 23.0
Net gain on disposal of assets		30.6
Asset Impairment Charges		0.0
Successor | Contract Termination [Member]
Other Commercial Events [Line Items]
Asset Impairment Charges		0.0
Successor | Red Mountain Joint Venture
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable		20.0
Net gain on disposal of assets		7.1
Successor | Queensland’s Bowen Basin [Member]
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable	$ 37.0
Net gain on disposal of assets		$ 20.6
Successor | DTA and PBGC
Other Commercial Events [Line Items]
Equity method investment, ownership percentage				50.00%
Predecessor
Other Commercial Events [Line Items]
Proceeds from disposal of assets, net of notes receivable			$ 24.3
Net gain on disposal of assets			22.8
Asset Impairment Charges			30.5
Predecessor | Contract Termination [Member]
Other Commercial Events [Line Items]
Asset Impairment Charges			$ 30.5
Predecessor | DTA and PBGC
Other Commercial Events [Line Items]
Equity method investment, ownership percentage			37.50%
Proceeds from disposal of assets, net of notes receivable			$ 20.5
Net gain on disposal of assets			$ 19.7