Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	$ (52.4)	$ 157.1
Net cash used in discontinued operations	(4.0)	(76.8)
Net cash (used in) provided by operating activities	(56.4)	80.3
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(50.9)	(64.7)
Changes in accrued expenses related to capital expenditures	(13.2)	(17.5)
Proceeds from disposal of assets, net of notes receivable	23.9	139.2
Purchases of debt and equity securities	(17.9)	(4.2)
Proceeds from sales and maturities of debt and equity securities	27.1	5.0
Contributions to joint ventures	(239.8)	(271.3)
Distributions from joint ventures	236.7	269.0
Other, net	(2.0)	(12.1)
Net cash (used in) provided by investing activities	(36.1)	43.4
Cash Flows From Financing Activities
Proceeds from long-term debt	975.7	0.0
Repayments of long-term debt	(660.7)	(10.4)
Payment of deferred financing costs	(28.7)	(10.1)
Dividends paid	(1.4)	(46.1)
Other, net	(3.3)	(2.7)
Transactions with affiliates, net	0.0	0.0
Net cash provided by (used in) financing activities	281.6	(69.3)
Net change in cash and cash equivalents	189.1	54.4
Cash and cash equivalents at beginning of period	298.0	444.0
Cash and cash equivalents at end of period	487.1	498.4
Parent Company [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	(367.0)	(112.4)
Net cash used in discontinued operations	(0.6)	(72.9)
Net cash (used in) provided by operating activities	(367.6)	(185.3)
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	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
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
Repayments of long-term debt	(656.0)	(6.0)
Payment of deferred financing costs	(28.7)	(10.1)
Dividends paid	(1.4)	(46.1)
Other, net	0.1	1.0
Transactions with affiliates, net	235.6	269.4
Net cash provided by (used in) financing activities	525.3	208.2
Net change in cash and cash equivalents	157.7	22.9
Cash and cash equivalents at beginning of period	188.7	300.7
Cash and cash equivalents at end of period	346.4	323.6
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	218.7	330.3
Net cash used in discontinued operations	(1.8)	(1.4)
Net cash (used in) provided by operating activities	216.9	328.9
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(37.4)	(32.1)
Changes in accrued expenses related to capital expenditures	(3.8)	(1.3)
Proceeds from disposal of assets, net of notes receivable	23.9	50.9
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
Distributions from joint ventures	0.0	0.0
Other, net	1.2	(1.7)
Net cash (used in) provided by investing activities	(16.1)	15.8
Cash Flows From Financing Activities
Proceeds from long-term debt	0.0
Repayments of long-term debt	(0.1)	0.0
Payment of deferred financing costs	0.0	0.0
Dividends paid	0.0	0.0
Other, net	(1.6)	(1.7)
Transactions with affiliates, net	(200.1)	(343.1)
Net cash provided by (used in) financing activities	(201.8)	(344.8)
Net change in cash and cash equivalents	(1.0)	(0.1)
Cash and cash equivalents at beginning of period	1.2	0.3
Cash and cash equivalents at end of period	0.2	0.2
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash (used in) provided by continuing operations	95.9	(60.8)
Net cash used in discontinued operations	(1.6)	(2.5)
Net cash (used in) provided by operating activities	94.3	(63.3)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(13.5)	(32.6)
Changes in accrued expenses related to capital expenditures	(9.4)	(16.2)
Proceeds from disposal of assets, net of notes receivable	0.0	88.3
Purchases of debt and equity securities	(17.9)	(4.2)
Proceeds from sales and maturities of debt and equity securities	27.1	5.0
Contributions to joint ventures	(239.8)	(271.3)
Distributions from joint ventures	236.7	269.0
Other, net	(3.2)	(10.4)
Net cash (used in) provided by investing activities	(20.0)	27.6
Cash Flows From Financing Activities
Proceeds from long-term debt	0.0
Repayments of long-term debt	(4.6)	(4.4)
Payment of deferred financing costs	0.0	0.0
Dividends paid	0.0	0.0
Other, net	(1.8)	(2.0)
Transactions with affiliates, net	(35.5)	73.7
Net cash provided by (used in) financing activities	(41.9)	67.3
Net change in cash and cash equivalents	32.4	31.6
Cash and cash equivalents at beginning of period	108.1	143.0
Cash and cash equivalents at end of period	$ 140.5	$ 174.6