Emergence from the Chapter 11 Cases and Fresh Start Reporting - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions	Apr. 01, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Effect of Plan
Fresh-Start Adjustment [Line Items]
Common stock, shares issued (in shares)		137.3
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		$ 78,400,000
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)	$ 22.02
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)		3.6
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
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%