Intangible Contract Assets and Liabilities - Balances and Balance Sheet Classifications (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fresh-Start Adjustment [Line Items]
Assets	$ 8.8	$ 20.7
Liabilities	(51.5)	(61.4)
Net Total	(42.7)	(40.7)
Investments and other assets
Fresh-Start Adjustment [Line Items]
Assets	8.8	20.7
Liabilities	0.0	0.0
Net Total	(8.8)	(20.7)
Accounts payable and accrued expenses
Fresh-Start Adjustment [Line Items]
Assets	0.0	0.0
Liabilities	(4.4)	(8.4)
Net Total	(4.4)	(8.4)
Other noncurrent liabilities
Fresh-Start Adjustment [Line Items]
Assets	0.0	0.0
Liabilities	(47.1)	(53.0)
Net Total	(47.1)	(53.0)
Coal supply agreements
Fresh-Start Adjustment [Line Items]
Liabilities	(18.1)	(21.4)
Take-or-pay contracts
Fresh-Start Adjustment [Line Items]
Liabilities	(33.4)	(40.0)
Net Total	(33.4)	(40.0)
Coal supply agreements
Fresh-Start Adjustment [Line Items]
Assets	8.8	20.7
Net Total	(9.3)	(0.7)
Take-or-pay contracts
Fresh-Start Adjustment [Line Items]
Assets	$ 0.0	$ 0.0

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 928
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6473545&loc=d3e61844-108004

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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+ References

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