Effect of Reorganization Plan and Fresh Start Adjustments on Balance Sheet (Details) $ in Millions	Apr. 01, 2017 USD ($)
Fresh-Start Adjustment, Increase (Decrease), Assets [Abstract]
Fresh-start adjustment, increase (decrease), restricted cash	$ (1,000.0)
Fresh-start adjustment, increase (decrease), inventories	70.1
Fresh-start adjustment, increase (decrease), other current assets	(333.0)
Fresh-start adjustment, increase (decrease), property, plant, equipment and mine development, net	(3,461.4)
Fresh-start adjustment, increase (decrease), Investments	238.0
Fresh-Start Adjustment, Increase (Decrease), Liabilities [Abstract]
Fresh-start adjustment, increase (decrease), accounts payable and accrued expenses	(14.8)
Fresh-Start Adjustment, Increase (Decrease), Long-term Debt	950.0
Fresh-start adjustment, increase (decrease), deferred income taxes	177.8
Fresh-start adjustment, increase (decrease), asset retirement obligations	73.9
Fresh-start adjustment, increase (decrease), accrued postretirement benefit costs	6.9
Fresh-start adjustment, increase (decrease), other noncurrent liabilities	(120.6)
Fresh-Start Adjustment, Increase (Decrease), Stockholders' Equity [Abstract]
Successor Common Stock	0.7
Fresh-Start Adjustment, Increase (Decrease), Preferred Stock	1,305.4
Successor Additional paid-in capital	(1,774.9)
Effect of Plan
Postconfirmation, Current Assets [Abstract]
Postconfirmation, total stockholders' equity	$ 3,105.0

Fresh-Start Adjustment, Increase (Decrease), Accounts Payable and Accrued Liabilities

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Fresh-Start Adjustment, Increase (Decrease), Asset Retirement Obligations

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Amount of fresh-start adjustment to additional paid-in capital.

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Amount of fresh-start adjustment to common stock.

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Amount of fresh-start adjustment to noncurrent deferred income tax liabilities.

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Amount of fresh-start adjustment to inventories.

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Amount of fresh-start adjustment to investments.

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Total fresh-start adjustment to long-term debt.

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Amount of fresh-start adjustment to noncurrent obligations classified as other.

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Amount of fresh-start adjustment to other postretirement obligations.

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Amount of fresh-start adjustment to preferred stock.

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Amount of fresh-start adjustment to prepaid and other current assets.

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Total fresh-start adjustment to property and equipment, net.

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Amount of fresh-start adjustment to current restricted cash and cash equivalents.

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Total stockholders' equity at the balance sheet date after fresh-start adjustments.

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