Reorganization Items, Net Additional Information (Details) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended	12 Months Ended
	Apr. 01, 2017 USD ($) years $ / shares shares	Apr. 01, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Fresh-Start Adjustment [Line Items]
Debtor Reorganization Items, Gain (Loss) on Settlement of Other Claims, Net	$ (3,031.2)	$ 3,031.2	$ 12.8
Property, Plant, and Equipment, Fair Value Disclosure	$ 3,461.4	$ 3,461.4
Asset Retirement Obligation			$ 750.2	$ 752.3	$ 691.1
Effect of Reorganization Plan [Member]
Fresh-Start Adjustment [Line Items]
Common Stock, shares issued (in shares) | shares	70.9	70.9
Common Stock, Par or Stated Value Per Share | $ / shares	$ 0.01	$ 0.01
Postconfirmation, Preferred Stock	$ 750.0	$ 750.0
Preferred Stock, Conversion, Discount Percent Of Common Stock Value	35.00%	35.00%
Postconfirmation, Common Stock, Initial Share Price Estimate | $ / shares	$ 25.00	$ 25.00
Dividends, Common Stock, Paid In Kind, Guaranteed Period | years	3
Dividends, Common Stock, Paid In Kind, Annual Stated Rate	8.50%	8.50%
Convertible Preferred Stock, Shares Issued upon Conversion | shares	46.2	46.2
Convertible Preferred Stock, Converted Shares, Equity Ownership Percent	42.00%	42.00%
Postconfirmation, Stockholders' Equity	$ 3,105.0	$ 3,105.0
Convertible Preferred Stock, Converted Shares, Fair Value	1,305.4	$ 1,305.4
Assets Held-for-sale, Long Lived, Fair Value Disclosure						$ 228.5
Write Off Of Mine Development Costs	89.5
Write Off Of Prepaid Assets	15.0
Increase (Decrease) in Obligation, Pension Benefits	$ 9.2
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.10%	4.10%	4.15%
Paid In Kind | Effect of Reorganization Plan [Member]
Fresh-Start Adjustment [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion | shares	13.1	13.1
Land and coal interests
Fresh-Start Adjustment [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	$ 3,504.7	$ 3,504.7
Asset Retirement Obligation	281.2	281.2
Buildings and improvements
Fresh-Start Adjustment [Line Items]
Postconfirmation, Property and Equipment, Net	466.1	466.1
Machinery and equipment
Fresh-Start Adjustment [Line Items]
Postconfirmation, Property and Equipment, Net	940.5	940.5
Coal supply agreements | Effect of Reorganization Plan [Member]
Fresh-Start Adjustment [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	314.9	314.9
Finite-Lived Intangible Assets, Fair Value Disclosure, Noncurrent	58.7	58.7
Contract-Based Intangible Assets | Effect of Reorganization Plan [Member]
Fresh-Start Adjustment [Line Items]
Finite-lived Intangible Assets, Fair Value Disclosure	32.6	32.6
Liabilities, Held For Sale						$ 15.7
Finite-Lived Intangible Assets, Fair Value Disclosure, Noncurrent	$ 83.6	$ 83.6
Australian Operations
Fresh-Start Adjustment [Line Items]
Credit adjusted risk free interest rates			4.36%
U.S. Obligations
Fresh-Start Adjustment [Line Items]
Credit adjusted risk free interest rates			9.36%