Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Parent Company [Member]	Mar. 31, 2013 Parent Company [Member]	Mar. 31, 2014 Guarantor Subsidiaries [Member]	Mar. 31, 2013 Guarantor Subsidiaries [Member]	Mar. 31, 2014 Non-Guarantor Subsidiaries [Member]	Mar. 31, 2013 Non-Guarantor Subsidiaries [Member]	Mar. 31, 2014 Consolidation, Eliminations [Member]	Dec. 31, 2013 Consolidation, Eliminations [Member]
Cash Flows From Operating Activities
Net cash provided by (used In) continuing operations		$ 126.9	$ 287.5	$ 108.5	$ 149.1	$ 49.6	$ 150.9	$ (31.2)	$ (12.5)
Net cash (used in) provided by discontinued operations		(72.8)	(15.8)	(72.0)	(4.4)	(1.1)	1.5	0.3	(12.9)
Net cash provided by (used in) operating activities		54.1	271.7	36.5	144.7	48.5	152.4	(30.9)	(25.4)
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(24.4)	(74.0)	0	0	(9.2)	(16.7)	(15.2)	(57.3)
Changes in accrued expenses related to capital expenditures		(18.3)	(66.4)	0	0	(1.1)	(2.0)	(17.2)	(64.4)
Proceeds from disposal of assets, net of notes receivable	62.6	99.8	53.0	0	0	12.7	1.8	87.1	51.2
Purchases of debt securities		(2.0)	(4.6)	0	0	0	0	(2.0)	(4.6)
Proceeds from sales and maturities of debt securities		0.4	12.9	0	0	0	0	0.4	12.9
Contributions to joint ventures		(151.8)	(154.1)	0	0	0	0	(151.8)	(154.1)
Distributions from joint ventures		138.2	174.3	0	0	0	0	138.2	174.3
Advances to related parties		(3.1)	(23.1)	0	0	0	0	(3.1)	(23.1)
Repayments of loans from related parties		1.5	14.8	0	0	0	0	1.5	14.8
Other, net		(0.6)	(1.0)	0	0	(0.6)	(1.0)	0	0
Net cash provided by (used in) continuing operations		39.7	(68.2)		0		(17.9)		(50.3)
Net cash used in discontinued operations		0	(0.1)		0		0		(0.1)
Net cash provided by (used in) investing activities		39.7	(68.3)	0	0	1.8	(17.9)	37.9	(50.4)
Cash Flows From Financing Activities
Repayments of long-term debt		(5.2)	(108.5)	(3.0)	(100.0)	0	0	(2.2)	(8.5)
Dividends paid		(23.1)	(22.9)	(23.1)	(22.9)	0	0	0	0
Repurchase of employee common stock relinquished for tax withholding		(2.0)	(2.6)	(2.0)	(2.6)	0	0	0	0
Other, net		0.6	1.3	3.1	3.4	(1.7)	(1.8)	(0.8)	(0.3)
Transactions with affiliates, net		0	0	(15.6)	40.3	(48.7)	(132.8)	64.3	92.5
Net cash (used in) provided by financing activities		(29.7)	(132.7)	(40.6)	(81.8)	(50.4)	(134.6)	61.3	83.7
Net change in cash and cash equivalents		64.1	70.7	(4.1)	62.9	(0.1)	(0.1)	68.3	7.9
Cash and cash equivalents at beginning of period	444.0	444.0	558.8	300.7	269.6	0.3	0.3	143.0	288.9	0	0
Cash and cash equivalents at end of period		$ 508.1	$ 629.5	$ 296.6	$ 332.5	$ 0.2	$ 0.2	$ 211.3	$ 296.8	$ 0	$ 0