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Financial Instruments and Other Guarantees - Textuals (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
	Apr. 01, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018	Dec. 31, 2017	Apr. 03, 2017
Successor
Guarantee Obligations [Line Items]
Surety Bonds outstanding		$ 1,587.9			$ 1,587.9
Letters of credit outstanding, amount		276.1			276.1
Asset retirement obligations		672.6			672.6	$ 657.0
Interest expense		38.3	$ 41.4		74.6
Collateral arrangements		0.0			0.0	323.1
Net impact on collateral requirements			49.7		323.1
Restricted cash						$ 40.1
Successor \| Secured Debt \| Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Letters of credit outstanding, amount		0.0			0.0
Letters of credit drawn on, amount		146.3			146.3
Line of Credit Facility, Maximum Borrowing Capacity							$ 250.0
Cash collateral required		0.0			0.0
Interest expense		1.9	$ 4.8		3.8
Successor \| U.S Mining
Guarantee Obligations [Line Items]
Surety Bonds outstanding		1,077.4			1,077.4
Letters of credit outstanding, amount		138.3			138.3
Asset retirement obligations		474.4			474.4
Successor \| Australian Mining
Guarantee Obligations [Line Items]
Surety Bonds outstanding		230.7			230.7
Letters of credit outstanding, amount		14.4			14.4
Asset retirement obligations		$ 229.8			$ 229.8
Predecessor
Guarantee Obligations [Line Items]
Interest expense	$ 0.0			$ 32.9
Net impact on collateral requirements				$ (66.4)
Predecessor \| Secured Debt \| Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Interest expense	$ 2.0