Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Deferred Tax Liabilities, Net, Current	$ 3.8
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	18.9	$ 441.0	$ 780.1
Net cash used in discontinued operations	(33.3)	(104.4)	(57.7)
Net cash (used in) provided by operating activities	(14.4)	336.6	722.4
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(126.8)	(194.4)	(328.4)
Changes in accrued expenses related to capital expenditures	(9.2)	(16.6)	(120.7)
Federal coal lease expenditures	277.2	276.7	276.8
Proceeds from disposal of assets, net of notes receivable	70.4	203.7	178.3
Purchases of debt and equity securities	(28.8)	(15.1)	(22.8)
Proceeds from sales and maturities of debt and equity securities	90.3	13.5	22.9
Maturity of short-term investments	0.0	0.0	4.8
Contributions to joint ventures	(425.4)	(529.8)	(671.7)
Distributions from joint ventures	422.6	534.2	722.9
Advances to related parties	(3.7)	(33.7)	(42.1)
Repayment of loans from related parties	0.9	5.4	25.2
Other, net	(3.1)	(5.0)	(5.8)
Net cash used in continuing operations	(290.0)	(314.5)	(514.2)
Net cash used in discontinued operations	0.0	0.0	(1.5)
Net cash used in investing activities	(290.0)	(314.5)	(515.7)
Cash Flows From Financing Activities
Proceeds from long-term debt	975.7	1.1	1,188.0
Repayments of long-term debt	(671.3)	(21.0)	(1,390.2)
Payment of deferred financing costs	(28.7)	(10.1)	(22.8)
Dividends paid	(1.4)	(92.3)	(91.7)
Restricted cash for distributions to noncontrolling interests	(2.0)	(42.5)
Restricted cash for distributions to noncontrolling interests	0.0	(42.5)	0.0
Other, net	(6.6)	(3.3)	(4.8)
Transactions with affiliates, net	0.0	0.0	0.0
Net cash provided by (used in) financing activities	267.7	(168.1)	(321.5)
Net change in cash and cash equivalents	(36.7)	(146.0)	(114.8)
Cash and cash equivalents at beginning of year	298.0	444.0	558.8
Cash and cash equivalents at end of year	261.3	298.0	444.0
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Deferred Tax Liabilities, Net, Current	11.8
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	(692.9)	(369.0)	(24.3)
Net cash used in discontinued operations	(27.4)	(73.3)	(20.4)
Net cash (used in) provided by operating activities	(720.3)	(442.3)	(44.7)
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	0.0
Proceeds from sales and maturities of debt and equity securities	0.0	0.0	0.0
Maturity of short-term investments			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 continuing operations			0.0
Net cash used in discontinued operations			0.0
Net cash used in investing activities	0.0	0.0	0.0
Cash Flows From Financing Activities
Proceeds from long-term debt	975.7	0.0	1,188.0
Repayments of long-term debt	(662.0)	(12.0)	(1,334.2)
Payment of deferred financing costs	(28.7)	(10.1)	(22.8)
Dividends paid	(1.4)	(92.3)	(91.7)
Restricted cash for distributions to noncontrolling interests		0.0
Other, net	1.4	3.1	4.2
Transactions with affiliates, net	253.8	441.6	332.3
Net cash provided by (used in) financing activities	538.8	330.3	75.8
Net change in cash and cash equivalents	(181.5)	(112.0)	31.1
Cash and cash equivalents at beginning of year	188.7	300.7	269.6
Cash and cash equivalents at end of year	7.2	188.7	300.7
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Deferred Tax Liabilities, Net, Current	0.0
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	593.5	776.1	778.7
Net cash used in discontinued operations	(2.9)	(4.6)	(7.6)
Net cash (used in) provided by operating activities	590.6	771.5	771.1
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(87.2)	(108.5)	(95.9)
Changes in accrued expenses related to capital expenditures	(3.6)	3.4	(1.2)
Federal coal lease expenditures	(277.2)	(276.7)	(276.8)
Proceeds from disposal of assets, net of notes receivable	36.3	105.9	93.0
Purchases of debt and equity securities	0.0	0.0	0.0
Proceeds from sales and maturities of debt and equity securities	0.0	0.0	0.0
Maturity of short-term investments			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	(3.2)	(4.4)	(5.7)
Net cash used in continuing operations			(286.6)
Net cash used in discontinued operations			0.0
Net cash used in investing activities	(334.9)	(280.3)	(286.6)
Cash Flows From Financing Activities
Proceeds from long-term debt	0.0	0.0	0.0
Repayments of long-term debt	(0.2)	(0.1)	(0.2)
Payment of deferred financing costs	0.0	0.0	0.0
Dividends paid	0.0	0.0	0.0
Restricted cash for distributions to noncontrolling interests		0.0
Other, net	(1.8)	(1.7)	(1.6)
Transactions with affiliates, net	(254.6)	(488.5)	(482.7)
Net cash provided by (used in) financing activities	(256.6)	(490.3)	(484.5)
Net change in cash and cash equivalents	(0.9)	0.9	0.0
Cash and cash equivalents at beginning of year	1.2	0.3	0.3
Cash and cash equivalents at end of year	0.3	1.2	0.3
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Deferred Tax Liabilities, Net, Current	3.8
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	118.3	33.9	25.7
Net cash used in discontinued operations	(3.0)	(26.5)	(29.7)
Net cash (used in) provided by operating activities	115.3	7.4	(4.0)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(39.6)	(85.9)	(232.5)
Changes in accrued expenses related to capital expenditures	(5.6)	(20.0)	(119.5)
Federal coal lease expenditures	0.0	0.0	0.0
Proceeds from disposal of assets, net of notes receivable	34.1	97.8	85.3
Purchases of debt and equity securities	(28.8)	(15.1)	(22.8)
Proceeds from sales and maturities of debt and equity securities	90.3	13.5	22.9
Maturity of short-term investments			4.8
Contributions to joint ventures	(425.4)	(529.8)	(671.7)
Distributions from joint ventures	422.6	534.2	722.9
Advances to related parties	(3.7)	(33.7)	(42.1)
Repayment of loans from related parties	0.9	5.4	25.2
Other, net	0.1	(0.6)	(0.1)
Net cash used in continuing operations			(227.6)
Net cash used in discontinued operations			(1.5)
Net cash used in investing activities	44.9	(34.2)	(229.1)
Cash Flows From Financing Activities
Proceeds from long-term debt	0.0	1.1	0.0
Repayments of long-term debt	(9.1)	(8.9)	(55.8)
Payment of deferred financing costs	0.0	0.0	0.0
Dividends paid	0.0	0.0	0.0
Restricted cash for distributions to noncontrolling interests		(42.5)
Other, net	(6.2)	(4.7)	(7.4)
Transactions with affiliates, net	0.8	46.9	150.4
Net cash provided by (used in) financing activities	(14.5)	(8.1)	87.2
Net change in cash and cash equivalents	145.7	(34.9)	(145.9)
Cash and cash equivalents at beginning of year	108.1	143.0	288.9
Cash and cash equivalents at end of year	253.8	108.1	$ 143.0
Consolidation, Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
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