Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Accounts Payable and Other Accrued Liabilities, Current	$ 984.2	$ 984.2	$ 1,442.5
Liabilities	11,596.2	11,596.2	10,195.2
Liabilities Subject to Compromise	8,440.2	8,440.2	0.0
Restricted Cash and Investments, Current	54.3	54.3	0.0
Deferred Tax Liabilities, Net, Current			3.8
Cash Flows From Operating Activities
Net cash provided by continuing operations	490.0	(22.9)	18.9	$ 441.0
Net cash used in discontinued operations	(29.2)	(29.9)	(33.3)	(104.4)
Net Cash Provided by (Used in) Operating Activities	460.8	(52.8)	(14.4)	336.6
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(106.7)	(126.6)	(126.8)	(194.4)
Changes in accrued expenses related to capital expenditures	(2.1)	(6.1)	(9.2)	(16.6)
Federal coal lease expenditures		(249.0)	277.2	276.7
Proceeds from disposal of assets, net of notes receivable	142.2	144.4	70.4	203.7
Purchases of debt and equity securities		0.0	(28.8)	(15.1)
Proceeds from sales and maturities of debt and equity securities		0.0	90.3	13.5
Contributions to joint ventures	(208.3)	(309.5)	(425.4)	(529.8)
Distributions from joint ventures	215.4	312.4	422.6	534.2
Advances to related parties	(39.3)	(40.4)	(3.7)	(33.7)
Repayment of loans from related parties	(39.8)	40.6	0.9	5.4
Other, net	(4.6)	(9.9)	(3.1)	(5.0)
Net cash used in continuing operations	(213.1)	(244.1)	(290.0)	(314.5)
Net cash used in investing activities		(244.1)	(290.0)	(314.5)
Cash Flows From Financing Activities
Proceeds from long-term debt	511.4	1,458.4	975.7	1.1
Repayments of long-term debt	(506.6)	(513.7)	(671.3)	(21.0)
Payment of deferred financing costs	(28.2)	(31.0)	(28.7)	(10.1)
Dividends paid		0.0	(1.4)	(92.3)
Restricted cash for distributions to noncontrolling interests			(2.0)
Restricted cash for distributions to noncontrolling interests		0.0	0.0	(42.5)
Other, net	(0.1)	(5.8)	(6.6)	(3.3)
Transactions with affiliates, net	0.0	0.0	0.0	0.0
Net cash provided by (used in) financing activities	(27.5)	907.9	267.7	(168.1)
Net change in cash and cash equivalents	220.2	611.0	(36.7)	(146.0)
Cash and cash equivalents at beginning of year	652.1	261.3	298.0	444.0
Cash and cash equivalents at end of year	872.3	872.3	261.3	298.0
Accounts Payable and Accrued Liabilities, Current	990.4	990.4	1,446.3
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Accounts Payable and Other Accrued Liabilities, Current	58.9	58.9
Liabilities	9,521.2	9,521.2	8,171.7
Liabilities Subject to Compromise	8,241.4	8,241.4
Restricted Cash and Investments, Current	13.8	13.8
Deferred Tax Liabilities, Net, Current			11.8
Cash Flows From Operating Activities
Net cash provided by continuing operations		(167.3)	(692.9)	(369.0)
Net cash used in discontinued operations		(16.2)	(27.4)	(73.3)
Net Cash Provided by (Used in) Operating Activities		(183.5)	(720.3)	(442.3)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		0.0	0.0	0.0
Changes in accrued expenses related to capital expenditures		0.0	0.0	0.0
Federal coal lease expenditures		0.0	0.0	0.0
Proceeds from disposal of assets, net of notes receivable		0.0	0.0	0.0
Purchases of debt and equity securities			0.0	0.0
Proceeds from sales and maturities of debt and equity securities			0.0	0.0
Contributions to joint ventures		0.0	0.0	0.0
Distributions from joint ventures		0.0	0.0	0.0
Advances to related parties		0.0	0.0	0.0
Repayment of loans from related parties		0.0	0.0	0.0
Other, net		0.0	0.0	0.0
Net cash used in investing activities		0.0	0.0	0.0
Cash Flows From Financing Activities
Proceeds from long-term debt		1,450.6	975.7	0.0
Repayments of long-term debt		(503.0)	(662.0)	(12.0)
Payment of deferred financing costs		(26.8)	(28.7)	(10.1)
Dividends paid			(1.4)	(92.3)
Restricted cash for distributions to noncontrolling interests				0.0
Other, net		0.0	1.4	3.1
Transactions with affiliates, net		(477.9)	253.8	441.6
Net cash provided by (used in) financing activities		442.9	538.8	330.3
Net change in cash and cash equivalents		259.4	(181.5)	(112.0)
Cash and cash equivalents at beginning of year		7.2	188.7	300.7
Cash and cash equivalents at end of year	266.6	266.6	7.2	188.7
Accounts Payable and Accrued Liabilities, Current			494.8
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Accounts Payable and Other Accrued Liabilities, Current	439.3	439.3
Liabilities	3,656.0	3,656.0	3,493.5
Liabilities Subject to Compromise	184.2	184.2
Restricted Cash and Investments, Current	0.0	0.0
Deferred Tax Liabilities, Net, Current			0.0
Cash Flows From Operating Activities
Net cash provided by continuing operations		78.5	615.3	764.7
Net cash used in discontinued operations		(1.9)	(2.9)	(4.6)
Net Cash Provided by (Used in) Operating Activities		76.6	612.4	760.1
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(55.5)	(70.6)	(95.8)
Changes in accrued expenses related to capital expenditures		(0.6)	(2.3)	2.2
Federal coal lease expenditures		(249.0)	(277.2)	(276.7)
Proceeds from disposal of assets, net of notes receivable		77.7	36.3	105.9
Purchases of debt and equity securities			0.0	0.0
Proceeds from sales and maturities of debt and equity securities			0.0	0.0
Contributions to joint ventures		0.0	0.0	0.0
Distributions from joint ventures		0.0	0.0	0.0
Advances to related parties		0.0	0.0	0.0
Repayment of loans from related parties		0.0	0.0	0.0
Other, net		(5.1)	(2.7)	(4.2)
Net cash used in investing activities		(232.5)	(316.5)	(268.6)
Cash Flows From Financing Activities
Proceeds from long-term debt		0.0	0.0	0.0
Repayments of long-term debt		(4.4)	(0.7)	(0.7)
Payment of deferred financing costs		0.0	0.0	0.0
Dividends paid			0.0	0.0
Restricted cash for distributions to noncontrolling interests				0.0
Other, net		(5.8)	(1.8)	(1.7)
Transactions with affiliates, net		268.4	(289.9)	(488.2)
Net cash provided by (used in) financing activities		258.2	(292.4)	(490.6)
Net change in cash and cash equivalents		102.3	3.5	0.9
Cash and cash equivalents at beginning of year		4.7	1.2	0.3
Cash and cash equivalents at end of year	107.0	107.0	4.7	1.2
Accounts Payable and Accrued Liabilities, Current			479.8
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Accounts Payable and Other Accrued Liabilities, Current	492.2	492.2
Liabilities	1,154.2	1,154.2	1,135.5
Liabilities Subject to Compromise	14.6	14.6
Restricted Cash and Investments, Current	40.5	40.5
Deferred Tax Liabilities, Net, Current			3.8
Cash Flows From Operating Activities
Net cash provided by continuing operations		65.9	96.5	45.3
Net cash used in discontinued operations		(11.8)	(3.0)	(26.5)
Net Cash Provided by (Used in) Operating Activities		54.1	93.5	18.8
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(71.1)	(56.2)	(98.6)
Changes in accrued expenses related to capital expenditures		(5.5)	(6.9)	(18.8)
Federal coal lease expenditures		0.0	0.0	0.0
Proceeds from disposal of assets, net of notes receivable		66.7	34.1	97.8
Purchases of debt and equity securities			(28.8)	(15.1)
Proceeds from sales and maturities of debt and equity securities			90.3	13.5
Contributions to joint ventures		(309.5)	(425.4)	(529.8)
Distributions from joint ventures		312.4	422.6	534.2
Advances to related parties		(40.4)	(3.7)	(33.7)
Repayment of loans from related parties		40.6	0.9	5.4
Other, net		(4.8)	(0.4)	(0.8)
Net cash used in investing activities		(11.6)	26.5	(45.9)
Cash Flows From Financing Activities
Proceeds from long-term debt		7.8	0.0	1.1
Repayments of long-term debt		(6.3)	(8.6)	(8.3)
Payment of deferred financing costs		(4.2)	0.0	0.0
Dividends paid			0.0	0.0
Restricted cash for distributions to noncontrolling interests				42.5
Other, net		0.0	(6.2)	(4.7)
Transactions with affiliates, net		209.5	36.1	46.6
Net cash provided by (used in) financing activities		206.8	21.3	(7.8)
Net change in cash and cash equivalents		249.3	141.3	(34.9)
Cash and cash equivalents at beginning of year		249.4	108.1	143.0
Cash and cash equivalents at end of year	498.7	498.7	249.4	$ 108.1
Accounts Payable and Accrued Liabilities, Current			467.9
Consolidation, Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Accounts Payable and Other Accrued Liabilities, Current	0.0	0.0
Liabilities	(2,735.2)	(2,735.2)	(2,605.5)
Liabilities Subject to Compromise	0.0	0.0
Restricted Cash and Investments, Current	0.0	0.0
Deferred Tax Liabilities, Net, Current			(11.8)
Cash Flows From Financing Activities
Cash and cash equivalents at beginning of year		0.0
Cash and cash equivalents at end of year	$ 0.0	$ 0.0	0.0
Accounts Payable and Accrued Liabilities, Current			$ 0.0