Accumulated Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Accumulated other comprehensive income [Line Items]
Foreign Currency Translation Adjustment	$ 23.3			$ 22.2
Reclassification from other comprehensive income to earnings	0
Current Period Change	1.1		12.7
Net actuarial loss associated with postretirement plans and workers' compensation obligations	(397.4)			(411.7)
Reclassification from other comprehensive income to earnings	14.3	[1]
Current period change	0
Prior service cost associated with postretirement plans	12.6			12.7
Reclassification from other comprehensive income to earnings	(0.1)	[1]
Current period change	0
Cash flow hedges	379.1			387.5
Net change in fair value	68.7		147.8
Reclassification from other comprehensive income to earnings	(77.1)	[1]	(84.2)
Available-for-sale securities	(6.0)			0.3
Net change in fair value	(6.2)		(11.5)
Reclassification from other comprehensive income to earnings	(0.1)	[1],[2]	0
Total accumulated other comprehensive income (loss)	11.6			11.0
Net change in fair value	62.5
Reclassification from other comprehensive income to earnings	(63.0)
Current period change	1.1
Cash flow hedges	91.8		130.3
Tax effect associated with the realized gain on sale of debt securities	0.1
Postretirement health care and life insurance benefits actuarial loss [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(6.0)	[1]
Defined benefit pension plans operating costs and expenses actuarial loss [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(13.6)	[1]
Defined benefit pension plans selling and administrative expenses actuarial loss [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(2.8)	[1]
Insignificant items actuarial loss [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(0.3)	[1]
Net actuarial loss associated with postretirement plans and workers compensation obligations total before income taxes [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(22.7)	[1]
Income tax provision actuarial loss [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	8.4	[1]
Postretirement health care and life insurance benefits prior service cost [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	0.4	[1]
Defined benefit pension plans operating costs and expenses prior service cost [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(0.2)	[1]
Prior service cost associated with postretirement plans total before income taxes [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	0.2	[1]
Income tax provision prior service cost [Member]
Accumulated other comprehensive income [Line Items]
Net actuarial loss and prior service cost associated with postretirement plans and workers compensation obligations	(0.1)	[1]
Cash Flow Hedging [Member]
Accumulated other comprehensive income [Line Items]
Cash flow hedges	85.1	[1]
Commodity Swaps and Options [Member]
Accumulated other comprehensive income [Line Items]
Cash flow hedges	6.9	[1]
Coal Trading [Member]
Accumulated other comprehensive income [Line Items]
Cash flow hedges	25.3	[1]
Insignificant items [Member]
Accumulated other comprehensive income [Line Items]
Cash flow hedges	(0.1)	[1]
Total before income taxes cash flow hedges [Member]
Accumulated other comprehensive income [Line Items]
Cash flow hedges	117.2	[1]
Income tax provision cash flow hedges [Member]
Accumulated other comprehensive income [Line Items]
Cash flow hedges	$ (40.1)	[1]

[1] 1) Recognized as gains (losses) in the unaudited condensed consolidated statements of operations.

[2] (2) The tax effect associated with the realized gain on sale of debt securities during the three months ended March 31, 2013 was less than $0.1 million.