Emergence from the Chapter 11 Cases and Fresh Start Reporting - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions	Apr. 01, 2017	Sep. 30, 2017	Jun. 30, 2017	Apr. 03, 2017	Mar. 31, 2017	Feb. 15, 2017	Dec. 31, 2016
Effect of Plan
Fresh-Start Adjustment [Line Items]
Common stock, shares issued (in shares)		134.8
Assets held-for-sale					$ 228,500,000
Write off of mine development costs	$ 89,500,000
Write off of prepaid assets	15,000,000
Successor
Fresh-Start Adjustment [Line Items]
Postconfirmation, preferred stock	1,305,400,000
Postconfirmation, total stockholders' equity	3,131,900,000
Postconfirmation, long-term debt	1,853,700,000
Debt issuance cost		$ 69,900,000
Successor | Effect of Plan
Fresh-Start Adjustment [Line Items]
Final equity value	$ 3,081,000,000
Postconfirmation, common stock, share price (in dollars per share)	$ 22.03
Common stock, shares issued (in shares)	70.9
Common stock, shares outstanding (in shares)	70.9
Common stock, par value per share (in dollars per share)	$ 0.01
Postconfirmation, preferred stock	$ 750,000,000
Discount percent of common stock value	35.00%
Postconfirmation, Common Stock, Initial Share Price Estimate	$ 25.00
Dividends, common stock, paid in kind, guaranteed period	3 years
Dividends, common stock, paid in kind, annual stated rate	8.50%
Convertible preferred stock, shares issued upon conversion	46.2
Equity ownership percent	42.00%
Postconfirmation, total stockholders' equity	$ 3,105,000,000
Convertible preferred stock, converted shares, fair value	$ 1,305,400,000
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	14.1
Incentive plan expiration period	10 years
Common stock, shares issued (in shares)		3.5
Minimum | Effect of Plan
Fresh-Start Adjustment [Line Items]
Final equity value	$ 4,200,000,000
Discounted cash flow, estimated weighed average cost of capital, percent	11.00%
Maximum | Effect of Plan
Fresh-Start Adjustment [Line Items]
Final equity value	$ 4,900,000,000
Discounted cash flow, estimated weighed average cost of capital, percent	14.00%
Restricted Stock | 2017 Incentive Plan | Effect of Plan
Fresh-Start Adjustment [Line Items]
Fair value of stock granted	$ 80,000,000
Paid In Kind | Successor | Effect of Plan
Fresh-Start Adjustment [Line Items]
Convertible preferred stock, shares issued upon conversion	13.1
Successor Credit Agreement | Term Loan | Effect of Plan
Fresh-Start Adjustment [Line Items]
Principal amount				$ 950,000,000
Successor Credit Agreement | Term Loan | Successor
Fresh-Start Adjustment [Line Items]
Principal amount			$ 650,000,000
6.00% Senior Secured Notes due March 2022 | Senior Notes | Effect of Plan
Fresh-Start Adjustment [Line Items]
Principal amount						$ 500,000,000.0
6.00% Senior Secured Notes due March 2022 | Senior Notes | Effect of Plan
Fresh-Start Adjustment [Line Items]
Principal amount						$ 500,000,000.0
Senior Secured Term Loan | Term Loan | Effect of Plan
Fresh-Start Adjustment [Line Items]
Postconfirmation, long-term debt	$ 950,000,000
Debt issuance cost	37,300,000
Successor Notes | Senior Notes | Effect of Plan
Fresh-Start Adjustment [Line Items]
Debt issuance cost	49,500,000
Principal amount	1,000,000,000.0
Coal supply agreements | Successor | Effect of Plan
Fresh-Start Adjustment [Line Items]
Finite-lived intangible assets, fair value	$ 314,900,000
Useful life	3 years
Finite-lived intangible assets, fair value, noncurrent	$ 58,700,000
Port And Rail Take Or Pay Contracts | Successor | 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
UNITED STATES | Asset Retirement Obligation | Successor | Effect of Plan
Fresh-Start Adjustment [Line Items]
Risk free interest rate	9.36%
AUSTRALIA | Asset Retirement Obligation | Successor | Effect of Plan
Fresh-Start Adjustment [Line Items]
Risk free interest rate	4.36%