Additional Information (Details) $ / shares in Units, shares in Millions	Apr. 01, 2017 USD ($) years $ / shares shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 shares	Apr. 03, 2017 USD ($)	Mar. 31, 2017 USD ($)	Feb. 15, 2017 USD ($)
Fresh-Start Adjustment [Line Items]
Common stock, shares outstanding (in shares) | shares	70.9	96.9	110.4	105.2
Debt issuance cost		$ 55,500,000	$ 68,900,000
Effect of Plan
Fresh-Start Adjustment [Line Items]
Common Stock, shares issued (in shares) | shares	70.9
Common Stock, par value per share (in dollars per share) | $ / shares	$ 0.01
Postconfirmation, preferred stock	$ 750,000,000.0
Discount percent of common stock value	35.00%
Postconfirmation, common stock, initial share price estimate | $ / shares	$ 25.00
Dividends, common stock, paid in kind, guaranteed period | years	3
Dividends, common stock, paid in kind, annual stated rate	8.50%
Convertible preferred stock, shares issued upon conversion | shares	46.2
Equity ownership percent	42.00%
Postconfirmation, total stockholders' equity	$ 3,105,000,000.0
Convertible preferred stock, converted shares, fair value	1,305,400,000
Assets held-for-sale						$ 228,500,000
Write off of mine development costs	89,500,000
Write off of prepaid assets	$ 15,000,000.0
Discount rate	4.10%		4.15%
Remeasurement reduction in pension liabilities	$ (9,200,000)
2017 Incentive Plan | Effect of Plan
Fresh-Start Adjustment [Line Items]
Common stock reserved for future issuance | shares	14.0
Paid In Kind | Effect of Plan
Fresh-Start Adjustment [Line Items]
Convertible preferred stock, shares issued upon conversion | shares	13.1
Successor Credit Agreement | Senior Secured Term Loan due 2025, net of original issue discount
Fresh-Start Adjustment [Line Items]
Principal amount		$ 400,000,000.0
Successor Credit Agreement | Senior Secured Term Loan due 2025, net of original issue discount | Effect of Plan
Fresh-Start Adjustment [Line Items]
Principal amount					$ 950,000,000.0
6.000% Senior Secured Notes due March 2022 | Senior Notes | Effect of Plan
Fresh-Start Adjustment [Line Items]
Principal amount							$ 500,000,000.0
6.375% Senior Secured Notes due March 2025 | Senior Notes | Effect of Plan
Fresh-Start Adjustment [Line Items]
Principal amount							$ 500,000,000.0
Coal supply agreements | Effect of Plan
Fresh-Start Adjustment [Line Items]
Finite-lived intangible assets, fair value	$ 314,900,000
Finite-lived intangible assets, fair value, noncurrent	58,700,000
Port And Rail Take Or Pay Contracts | Effect of Plan
Fresh-Start Adjustment [Line Items]
Finite-lived intangible assets, fair value	32,600,000
Liabilities, held-for-sale						$ 15,700,000
Finite-lived intangible assets, fair value, noncurrent	$ 83,600,000
U.S. Obligations
Fresh-Start Adjustment [Line Items]
Credit adjusted risk free interest rates			9.36%
Australian Operations
Fresh-Start Adjustment [Line Items]
Credit adjusted risk free interest rates			4.36%