Financial Instruments and Other Guarantees - Textuals (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Apr. 03, 2017
Successor
Guarantee Obligations [Line Items]
Asset retirement obligations	$ 665.9		$ 657.0
Letters of credit outstanding, amount	0.0
Interest expense	36.3
Collateral arrangements - current	109.1		0.0
Collateral arrangements	0.0		323.1
Net impact on collateral requirements	214.0
Restricted cash			$ 40.1
Successor | Secured Debt | Accounts Receivable Securitization Program, April 3, 2020
Guarantee Obligations [Line Items]
Letters of credit drawn on, amount	176.3
Line of Credit Facility, Maximum Borrowing Capacity				$ 250.0
Cash collateral required	0.0
Interest expense	1.3
Successor | U.S Mining
Guarantee Obligations [Line Items]
Asset retirement obligations	469.0
Surety Bonds outstanding for reclamation	1,074.3
Letters of credit outstanding, amount	174.1
Successor | Australian Mining
Guarantee Obligations [Line Items]
Asset retirement obligations	230.2
Surety Bonds outstanding for reclamation	270.1
Letters of credit outstanding, amount	14.4
Cash collateral in support of reclamation obligations or activities	$ 99.7
Predecessor
Guarantee Obligations [Line Items]
Interest expense		$ 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