Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2015
Cash Flows From Operating Activities
Net cash used in continuing operations	$ 85.2	$ (427.6)	$ (52.4)
Net cash (used in) provided by discontinued operations	(3.5)	(4.2)	(4.0)
Net cash used in operating activities	81.7	(431.8)	(56.4)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(18.2)	(38.1)	(50.9)
Changes in accrued expenses related to capital expenditures	(3.1)	(7.1)	(13.2)
Proceeds from disposal of assets, net of notes receivable	113.8	116.0	23.9
Purchases of debt and equity securities		0.0	(17.9)
Proceeds from sales and maturities of debt and equity securities		0.0	27.1
Contributions to joint ventures	(58.5)	(159.7)	(239.8)
Distributions from joint ventures	66.5	163.5	236.7
Other, net	(3.8)	(8.9)	(2.0)
Net cash provided by (used in) investing activities		65.7	(36.1)
Cash Flows From Financing Activities
Proceeds from long-term debt	475.0	1,422.0	975.7
Repayments of long-term debt	(1.9)	(9.0)	(660.7)
Payment of deferred financing costs	(26.7)	(29.5)	(28.7)
Dividends paid		0.0	(1.4)
Distributions to noncontrolling interests	(2.5)	(2.5)	(1.8)
Other, net	(0.1)	(1.9)	(1.5)
Transactions with affiliates, net	0.0	0.0	0.0
Net cash provided by (used in) financing activities	443.8	1,379.1	281.6
Net change in cash and cash equivalents	622.2	1,013.0	189.1
Cash and cash equivalents at beginning of period	652.1	261.3	298.0
Cash and cash equivalents at end of period	1,274.3	1,274.3	487.1
Parent Company [Member]
Cash Flows From Operating Activities
Net cash used in continuing operations		(272.1)	(367.0)
Net cash (used in) provided by discontinued operations		(3.7)	(0.6)
Net cash used in operating activities		(275.8)	(367.6)
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
Proceeds from disposal of assets, net of notes receivable		0.0	0.0
Purchases of debt and equity securities			0.0
Proceeds from sales and maturities of debt and equity securities			0.0
Contributions to joint ventures		0.0	0.0
Distributions from joint ventures		0.0	0.0
Other, net		0.0	0.0
Net cash provided by (used in) investing activities		0.0	0.0
Cash Flows From Financing Activities
Proceeds from long-term debt		1,422.0	975.7
Repayments of long-term debt		(3.0)	(656.0)
Payment of deferred financing costs		(25.6)	(28.7)
Dividends paid			(1.4)
Distributions to noncontrolling interests		0.0	0.0
Other, net		0.0	0.1
Transactions with affiliates, net		(387.2)	235.6
Net cash provided by (used in) financing activities		1,006.2	525.3
Net change in cash and cash equivalents		730.4	157.7
Cash and cash equivalents at beginning of period		7.2	188.7
Cash and cash equivalents at end of period	737.6	737.6	346.4
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash used in continuing operations		(32.0)	187.4
Net cash (used in) provided by discontinued operations		0.8	(1.8)
Net cash used in operating activities		(31.2)	185.6
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(21.4)	(26.5)
Changes in accrued expenses related to capital expenditures		(1.1)	(3.8)
Proceeds from disposal of assets, net of notes receivable		65.8	23.9
Purchases of debt and equity securities			0.0
Proceeds from sales and maturities of debt and equity securities			0.0
Contributions to joint ventures		0.0	0.0
Distributions from joint ventures		0.0	0.0
Other, net		(6.1)	(1.3)
Net cash provided by (used in) investing activities		37.2	(7.7)
Cash Flows From Financing Activities
Proceeds from long-term debt		0.0	0.0
Repayments of long-term debt		(3.7)	(0.4)
Payment of deferred financing costs		0.0	0.0
Dividends paid			0.0
Distributions to noncontrolling interests		0.0	0.0
Other, net		(1.9)	(1.6)
Transactions with affiliates, net		(2.0)	(177.5)
Net cash provided by (used in) financing activities		(7.6)	(179.5)
Net change in cash and cash equivalents		(1.6)	(1.6)
Cash and cash equivalents at beginning of period		4.7	3.6
Cash and cash equivalents at end of period	3.1	3.1	2.0
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash used in continuing operations		(123.5)	127.2
Net cash (used in) provided by discontinued operations		(1.3)	(1.6)
Net cash used in operating activities		(124.8)	125.6
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(16.7)	(24.4)
Changes in accrued expenses related to capital expenditures		(6.0)	(9.4)
Proceeds from disposal of assets, net of notes receivable		50.2	0.0
Purchases of debt and equity securities			(17.9)
Proceeds from sales and maturities of debt and equity securities			27.1
Contributions to joint ventures		(159.7)	(239.8)
Distributions from joint ventures		163.5	236.7
Other, net		(2.8)	(0.7)
Net cash provided by (used in) investing activities		28.5	(28.4)
Cash Flows From Financing Activities
Proceeds from long-term debt		0.0	0.0
Repayments of long-term debt		(2.3)	(4.3)
Payment of deferred financing costs		(3.9)	0.0
Dividends paid			0.0
Distributions to noncontrolling interests		(2.5)	(1.8)
Other, net		0.0	0.0
Transactions with affiliates, net		389.2	(58.1)
Net cash provided by (used in) financing activities		380.5	(64.2)
Net change in cash and cash equivalents		284.2	33.0
Cash and cash equivalents at beginning of period		249.4	105.7
Cash and cash equivalents at end of period	$ 533.6	$ 533.6	$ 138.7