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, 2018	Dec. 31, 2016
Successor | Advance royalty recoupment reserve
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period			$ 0.0
Charged to Costs and Expenses		$ 0.0	0.3
Charged to Other Accounts		0.0	0.0
Deductions(1)		0.0	0.0
Other		0.0	0.0
Balance at End of Period		0.0	0.3
Successor | Reserve for materials and supplies
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period			0.6
Charged to Costs and Expenses		1.0	0.5
Charged to Other Accounts		0.0	0.0
Deductions(1)		(0.4)	(0.9)
Other		0.0	0.0
Balance at End of Period		0.6	0.2
Successor | Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period			4.6
Charged to Costs and Expenses		4.6	(0.2)
Charged to Other Accounts		0.0	0.0
Deductions(1)		0.0	0.0
Other		0.0	0.0
Balance at End of Period		4.6	4.4
Successor | Tax valuation allowances
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period			2,432.5
Charged to Costs and Expenses		(744.9)	(275.0)
Charged to Other Accounts		0.0	0.0
Deductions(1)		0.0	0.0
Other		(111.0)	(63.2)
Balance at End of Period		2,432.5	$ 2,094.3
Predecessor | Advance royalty recoupment reserve
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 7.8	0.0		$ 8.3
Charged to Costs and Expenses	0.0			0.5
Charged to Other Accounts	(7.4)			0.0
Deductions(1)	(0.4)			(1.0)
Other	0.0			0.0
Balance at End of Period	0.0			7.8
Predecessor | Reserve for materials and supplies
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	5.6	0.0		4.7
Charged to Costs and Expenses	0.5			4.3
Charged to Other Accounts	(6.1)			0.0
Deductions(1)	0.0			(3.4)
Other	0.0			0.0
Balance at End of Period	0.0			5.6
Predecessor | Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	13.1	0.0		6.6
Charged to Costs and Expenses	0.0			7.9
Charged to Other Accounts	(12.8)			0.0
Deductions(1)	(0.3)			(1.4)
Other	0.0			0.0
Balance at End of Period	0.0			13.1
Predecessor | Tax valuation allowances
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	4,037.5	$ 3,288.4		1,614.1
Charged to Costs and Expenses	(777.2)			2,453.9
Charged to Other Accounts	28.1			0.0
Deductions(1)	0.0			0.0
Other	0.0			(30.5)
Balance at End of Period	$ 3,288.4			$ 4,037.5