Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended	12 Months Ended
		Apr. 01, 2017		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Advance royalty recoupment reserve [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Other Accounts		$ (7.4)	[1]		$ 0.0		$ 0.0
Reserve for materials and supplies [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Other Accounts		(6.1)	[1]		0.0		0.0
Allowance for Doubtful Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Other Accounts		(12.8)	[1]		0.0		0.0
Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Other Accounts		28.1	[1]		0.0		0.0
Predecessor | Advance royalty recoupment reserve [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense		0.0			0.5		0.0
Deductions		(0.4)	[2]		(1.0)	[3]	(0.9)	[3]
Other		0.0			0.0		1.6	[4]
Valuation Allowances and Reserves, Balance		0.0			7.8		8.3		$ 7.6
Predecessor | Reserve for materials and supplies [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense		0.5			4.3		0.4
Deductions	[2]	0.0			(3.4)		(0.3)
Other		0.0			0.0		0.0
Valuation Allowances and Reserves, Balance		0.0			5.6		4.7		4.6
Predecessor | Allowance for Doubtful Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense		0.0			7.9		8.0
Deductions	[2]	(0.3)			(1.4)		(7.2)
Other		0.0			0.0		0.0
Valuation Allowances and Reserves, Balance		0.0			13.1		6.6		5.8
Predecessor | Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense		(777.2)			2,453.9		452.9
Deductions	[2]	0.0			0.0		0.0
Other		0.0			(30.5)	[5]	(205.3)	[5]
Valuation Allowances and Reserves, Balance		3,288.4			$ 4,037.5		$ 1,614.1		$ 1,366.5
Successor | Advance royalty recoupment reserve [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense				$ 0.0
Charges to Other Accounts				0.0
Deductions	[2]			0.0
Other				0.0
Valuation Allowances and Reserves, Balance		0.0		0.0
Successor | Reserve for materials and supplies [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense				1.0
Charges to Other Accounts				0.0
Deductions	[2]			(0.4)
Other				0.0
Valuation Allowances and Reserves, Balance		0.0		0.6
Successor | Allowance for Doubtful Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense				4.6
Charges to Other Accounts				0.0
Deductions	[2]			0.0
Other				0.0
Valuation Allowances and Reserves, Balance		0.0		4.6
Successor | Valuation Allowance of Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Charges to Cost and Expense				(744.9)
Charges to Other Accounts				0.0
Deductions	[2]			0.0
Other	[6]			(111.0)
Valuation Allowances and Reserves, Balance		$ 3,288.4		$ 2,432.5

[1] Fresh start reporting adjustments.

[2] Reserves utilized, unless otherwise indicated.

[3] Deductions to advance royalty recoupment reserve represents the termination of federal and state leases.

[4] Balances transferred from other accounts.

[5] Includes the impact of the decrease in Australian dollar exchange rates.

[6] Release of valuation allowance primarily related to carrybacks of U.S. net operating losses.