Derivative Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 MMBTU	Mar. 31, 2012	Dec. 31, 2012 MMBTU
Derivative financial instruments
Unrealized mark-to-market gains (losses)	$ 3,900,000	$ (11,300,000)
Open derivative contracts for natural gas (in MMBtus)	7,200,000		9,900,000
Percentage of natural gas consumption covered by derivatives	91.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	0		800,000
Total derivative assets
Gross and Net amounts presented in consolidated balance sheet	3,100,000		200,000
Gross amounts not offset in consolidated balance sheet
Financial instruments			200,000
Cash collateral received (pledged)	0		0
Net amount	3,100,000
Total derivative liabilities
Gross and Net amounts presented in consolidated balance sheet			1,000,000
Gross amounts not offset in consolidated balance sheet
Financial instruments			200,000
Cash collateral received (pledged)	0		0
Net amount			800,000
Net assets (liabilities)
Gross and Net amounts presented in consolidated balance sheet	3,100,000		(800,000)
Exposure to credit loss from nonperformance by counterparties	3,100,000		0
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets	3,100,000		200,000
Unrealized losses in other current liabilities			(1,000,000)
Net unrealized derivative gains (losses)	3,100,000		(800,000)
Unrealized mark-to-market gains (losses)	3,700,000	(11,300,000)
Realized gains (losses)	(1,600,000)	(7,000,000)
Net derivative gains (losses)	$ 2,100,000	$ (18,300,000)