Investments Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Apr. 01, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Predecessor
Schedule of Equity Method Investments [Line Items]
Equity Method Investments						$ 85.3		$ 85.3
(Income) loss from equity affiliates				$ (15.0)	$ (15.0)	(28.8)		(16.2)	$ 15.9
Predecessor \| Interest In Middlemount Coal Pty Limited [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments						84.8		84.8
(Income) loss from equity affiliates				(17.4)				(22.6)	7.0
Predecessor \| Other Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments						$ 0.5		0.5
(Income) loss from equity affiliates				$ 2.4				$ 6.4	$ 8.9
Successor
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 83.8						$ 83.8
(Income) loss from equity affiliates	(22.8)	$ (10.5)	$ (15.7)				(49.0)
Successor \| Interest In Middlemount Coal Pty Limited [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	82.1						82.1
(Income) loss from equity affiliates							(48.6)
Successor \| Other Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 1.7						1.7
(Income) loss from equity affiliates							$ (0.4)