Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	$ (62.0)	$ 345.8
Net cash used in discontinued operations	(28.6)	(95.7)
Net cash (used in) provided by operating activities	(90.6)	250.1
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(76.9)	(107.5)
Changes in accrued expenses related to capital expenditures	(14.5)	(22.8)
Federal coal lease expenditures	(89.8)	(89.4)
Proceeds from disposal of assets, net of notes receivable	39.6	152.5
Purchases of debt and equity securities	(22.6)	(6.0)
Proceeds from sales and maturities of debt and equity securities	33.6	7.7
Contributions to joint ventures	(349.8)	(410.4)
Distributions from joint ventures	339.8	408.5
Advances to related parties	(3.6)	(19.7)
Other, net	(2.1)	(2.5)
Net cash (used in) provided by investing activities	(146.3)	(89.6)
Cash Flows From Financing Activities
Proceeds from long-term debt	975.7	1.1
Repayments of long-term debt	(666.0)	(15.6)
Payment of deferred financing costs	(28.7)	(10.1)
Dividends paid	(1.4)	(69.2)
Restricted cash for distributions to noncontrolling interests	0.0	(42.5)
Other, net	(6.4)	(1.7)
Transactions with affiliates, net	0.0	0.0
Net cash provided by (used in) financing activities	273.2	(138.0)
Net change in cash and cash equivalents	36.3	22.5
Cash and cash equivalents at beginning of period	298.0	444.0
Cash and cash equivalents at end of period	334.3	466.5
Parent Company [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	(649.0)	(286.9)
Net cash used in discontinued operations	(23.2)	(72.1)
Net cash (used in) provided by operating activities	(672.2)	(359.0)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	0.0	0.0
Changes in accrued expenses related to capital expenditures	0.0	0.0
Federal coal lease expenditures	0.0	0.0
Proceeds from disposal of assets, net of notes receivable	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
Distributions from joint ventures	0.0	0.0
Advances to related parties	0.0	0.0
Other, net	0.0	0.0
Net cash (used in) provided by investing activities	0.0	0.0
Cash Flows From Financing Activities
Proceeds from long-term debt	975.7	0.0
Repayments of long-term debt	(659.0)	(9.0)
Payment of deferred financing costs	(28.7)	(10.1)
Dividends paid	(1.4)	(69.2)
Restricted cash for distributions to noncontrolling interests		0.0
Other, net	1.4	3.2
Transactions with affiliates, net	339.7	460.8
Net cash provided by (used in) financing activities	627.7	375.7
Net change in cash and cash equivalents	(44.5)	16.7
Cash and cash equivalents at beginning of period	188.7	300.7
Cash and cash equivalents at end of period	144.2	317.4
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	499.8	634.2
Net cash used in discontinued operations	(2.8)	(3.6)
Net cash (used in) provided by operating activities	497.0	630.6
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(55.0)	(57.8)
Changes in accrued expenses related to capital expenditures	(3.4)	0.9
Federal coal lease expenditures	(89.8)	(89.4)
Proceeds from disposal of assets, net of notes receivable	7.6	54.7
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
Distributions from joint ventures	0.0	0.0
Advances to related parties	0.0	0.0
Other, net	(2.4)	(3.6)
Net cash (used in) provided by investing activities	(143.0)	(95.2)
Cash Flows From Financing Activities
Proceeds from long-term debt	0.0	0.0
Repayments of long-term debt	(0.1)	(0.1)
Payment of deferred financing costs	0.0	0.0
Dividends paid	0.0	0.0
Restricted cash for distributions to noncontrolling interests		0.0
Other, net	(1.6)	(1.6)
Transactions with affiliates, net	(353.2)	(533.8)
Net cash provided by (used in) financing activities	(354.9)	(535.5)
Net change in cash and cash equivalents	(0.9)	(0.1)
Cash and cash equivalents at beginning of period	1.2	0.3
Cash and cash equivalents at end of period	0.3	0.2
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	87.2	(1.5)
Net cash used in discontinued operations	(2.6)	(20.0)
Net cash (used in) provided by operating activities	84.6	(21.5)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(21.9)	(49.7)
Changes in accrued expenses related to capital expenditures	(11.1)	(23.7)
Federal coal lease expenditures	0.0	0.0
Proceeds from disposal of assets, net of notes receivable	32.0	97.8
Purchases of debt and equity securities	(22.6)	(6.0)
Proceeds from sales and maturities of debt and equity securities	33.6	7.7
Contributions to joint ventures	(349.8)	(410.4)
Distributions from joint ventures	339.8	408.5
Advances to related parties	(3.6)	(19.7)
Other, net	0.3	1.1
Net cash (used in) provided by investing activities	(3.3)	5.6
Cash Flows From Financing Activities
Proceeds from long-term debt	0.0	1.1
Repayments of long-term debt	(6.9)	(6.5)
Payment of deferred financing costs	0.0	0.0
Dividends paid	0.0	0.0
Restricted cash for distributions to noncontrolling interests		(42.5)
Other, net	(6.2)	(3.3)
Transactions with affiliates, net	13.5	73.0
Net cash provided by (used in) financing activities	0.4	21.8
Net change in cash and cash equivalents	81.7	5.9
Cash and cash equivalents at beginning of period	108.1	143.0
Cash and cash equivalents at end of period	$ 189.8	$ 148.9