Intangible Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2010 Coal supply agreement	Dec. 31, 2012 Coal supply agreement	Dec. 31, 2011 Coal supply agreement	Dec. 31, 2012 Other intangible assets	Aug. 31, 2012 Reed Minerals	Aug. 31, 2012 Reed Minerals Coal supply agreement	Aug. 31, 2012 Reed Minerals Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Cost of acquired entity								$ 73.3
Identifiable intangible assets								8.2	7.3	0.9
Goodwill	6.4	0						6.4
Finite-Lived Intangible Assets, Net [Abstract]
Gross carrying amount	92.4				91.5	84.2	0.9
Accumulated Amortization	(29.0)				(29.0)	(26.3)	0
Net Balance	63.4	57.9			62.5	57.9	0.9
Intangible asset impairment charge				0.7
Intangible assets amortization expense	2.8	2.1	3.5
Expected annual amortization expense, 2013	3.8
Expected annual amortization expense, 2014	3.6
Expected annual amortization expense, 2015	3.8
Expected annual amortization expense, 2016	3.8
Expected annual amortization expense, 2017	$ 3.8
Coal supply agreement amortization period	28 years