Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 MMBTU	Mar. 31, 2013	Dec. 31, 2013 MMBTU
Derivative financial instruments
Unrealized mark-to-market (losses) gains	$ (2.9)	$ 3.9
Open derivative contracts for natural gas (in MMBtus)	9,400,000		13,200,000
Percentage of natural gas consumption covered by derivatives	74.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	0.1		0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	4.9		7.8
Gross amounts not offset in consolidated balance sheet
Net amount	4.9		7.8
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	4.9		7.8
Gross amounts not offset in consolidated balance sheet
Cash collateral pledged	0		0
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets	4.9		7.8
Net unrealized derivative gains/(losses)	4.9		7.8
Unrealized mark-to-market (losses) gains	(4.0)	3.7
Realized gains (losses)	9.9	(1.6)
Net derivative gains/(losses)	$ 5.9	$ 2.1