Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative financial instruments
Unrealized mark-to-market gains (losses)			$ (0.7)	$ 9.6
Open derivative contracts for natural gas (in MMBtus)	11,300,000		11,300,000		9,900,000
Percentage of natural gas consumption covered by derivatives	90.00%		90.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	1.5		1.5		0.8
Total derivative assets
Amounts presented in consolidated balance sheet					0.2
Gross amounts not offset in consolidated balance sheet
Financial instruments					0.2
Cash collateral received (pledged)	0		0		0
Net amount					0
Total derivative liabilities
Amounts presented in consolidated balance sheet	1.5		1.5		1.0
Gross amounts not offset in consolidated balance sheet
Financial instruments					0.2
Cash collateral received (pledged)	0		0		0
Net amount	1.5		1.5		0.8
Net assets (liabilities)
Amounts presented in consolidated balance sheet	(1.5)		(1.5)		(0.8)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets					0.2
Unrealized losses in other current liabilities	(1.5)		(1.5)		(1.0)
Net unrealized derivative losses	(1.5)		(1.5)		(0.8)
Unrealized mark-to-market gains (losses)	(0.9)	7.1	(0.1)	9.6
Realized gains (losses)	(1.2)	(5.7)	(0.1)	(22.2)
Net derivative gains (losses)	$ (2.1)	$ 1.4	$ (0.2)	$ (12.6)