Derivative Financial Instruments (Details) MMBTU in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) MMBTU	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) MMBTU	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) MMBTU
Derivative financial instruments
Unrealized mark-to-market gains (losses)			$ 5,600,000	$ (7,600,000)
Open derivative contracts for natural gas (in MMBtus) | MMBTU	40.0		40.0		7.6
Percentage of natural gas consumption covered by derivatives			72.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 1,100,000		$ 1,100,000		$ 3,600,000
Cash Collateral for Borrowed Securities	0		0		0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	3,100,000		3,100,000		100,000
Gross amounts not offset in consolidated balance sheet
Financial instruments	1,100,000		1,100,000		100,000
Cash collateral received	0		0		0
Net amount	2,000,000		2,000,000		0
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	1,100,000		1,100,000		3,600,000
Gross amounts not offset in consolidated balance sheet
Financial instruments	1,100,000		1,100,000		100,000
Cash collateral pledged	0		0		0
Net amount	0		0		3,500,000
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	2,000,000		2,000,000		(3,500,000)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Derivative Asset, Current	1,900,000		1,900,000		100,000
Unrealized gains in other current assets	3,100,000		3,100,000		100,000
Derivative Liability, Current	(500,000)		(500,000)		(3,600,000)
Derivative Asset, Noncurrent	1,200,000		1,200,000		0
Unrealized losses in other current liabilities	(1,100,000)		(1,100,000)		(3,600,000)
Derivative Liability, Noncurrent	600,000		600,000		0
Net derivative assets (liabilities)	2,000,000		2,000,000		$ (3,500,000)
Unrealized mark-to-market gains (losses)	2,600,000	$ (4,900,000)	6,500,000	(8,900,000)
Realized (losses) gains	(800,000)	4,900,000	(5,400,000)	14,800,000
Net derivative gains	$ 1,800,000	$ 0	$ 1,100,000	$ 5,900,000