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Investments Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Apr. 01, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Successor
Schedule of Equity Method Investments [Line Items]
Equity Method Investments				$ 83.8					$ 83.8	$ 45.9
Income from equity affiliates	$ (17.2)	$ (25.2)	$ (22.0)	(22.8)	$ (10.5)	$ (15.7)			(49.0)	(68.1)
Successor | Equity method investment and financing receivables related to Middlemount
Schedule of Equity Method Investments [Line Items]
Equity Method Investments				82.1					82.1	45.0
Income from equity affiliates									(48.6)	(69.3)
Successor | Other equity method investments
Schedule of Equity Method Investments [Line Items]
Equity Method Investments				$ 1.7					1.7	0.9
Income from equity affiliates									$ (0.4)	$ 1.2
Predecessor
Schedule of Equity Method Investments [Line Items]
Income from equity affiliates							$ (15.0)	$ (15.0)			$ (16.2)
Predecessor | Equity method investment and financing receivables related to Middlemount
Schedule of Equity Method Investments [Line Items]
Income from equity affiliates							(17.4)				(22.6)
Predecessor | Other equity method investments
Schedule of Equity Method Investments [Line Items]
Income from equity affiliates							$ 2.4				$ 6.4

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

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-Publisher FASB
-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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