Financial Instruments and Other Guarantees - Textuals (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Apr. 01, 2017	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Apr. 03, 2017
Successor
Guarantee Obligations [Line Items]
Surety Bonds outstanding	$ 1,637.3				$ 1,637.3
Letters of credit outstanding, amount	252.2				252.2
Asset retirement obligations	670.7				670.7	$ 657.0
Interest expense	38.2	$ 42.4		$ 83.8	112.8
Collateral arrangements	0.0				0.0	323.1
Net impact on collateral requirements				81.2	323.1
Restricted cash						$ 40.1
Provision for North Goonyella equipment loss	49.3	$ 0.0		0.0	49.3
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	1.7			$ 3.9	5.5
Successor | U.S Mining
Guarantee Obligations [Line Items]
Surety Bonds outstanding	1,370.9				1,370.9
Letters of credit outstanding, amount	152.7				152.7
Asset retirement obligations	703.0				$ 703.0
Predecessor
Guarantee Obligations [Line Items]
Interest expense			$ 32.9
Net impact on collateral requirements			(66.4)
Provision for North Goonyella equipment loss			0.0
Predecessor | Secured Debt | Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Interest expense			$ 2.0
Leased Equipment | Successor
Guarantee Obligations [Line Items]
Provision for North Goonyella equipment loss	$ 40.2