Derivative Financial Instruments (Details) MMBTU in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) MMBTU	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MMBTU	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) MMBTU
Derivative financial instruments
Unrealized mark-to-market (losses) gains			$ (11,600,000)	$ (4,700,000)
Open derivative contracts for natural gas (in MMBtus) | MMBTU	67.2		67.2		7.6
Percentage of natural gas consumption covered by derivatives			63.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 15,500,000		$ 15,500,000		$ 3,600,000
Cash Collateral for Borrowed Securities	0		0		0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	300,000		300,000		100,000
Gross amounts not offset in consolidated balance sheet
Financial instruments	300,000		300,000		100,000
Cash collateral received	0		0		0
Net amount	0		0		0
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	(15,500,000)		(15,500,000)		(3,600,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments	(300,000)		(300,000)		(100,000)
Cash collateral pledged	0		0		0
Net amount	(15,200,000)		(15,200,000)		(3,500,000)
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	(15,200,000)		(15,200,000)		(3,500,000)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Derivative Asset, Current	0		0		100,000
Unrealized gains in other current assets	300,000		300,000		100,000
Derivative Liability, Current	(7,300,000)		(7,300,000)		(3,600,000)
Derivative Asset, Noncurrent	300,000		300,000		0
Unrealized losses in other current liabilities	(15,500,000)		(15,500,000)		(3,600,000)
Derivative Liability, Noncurrent	8,200,000		8,200,000		0
Net derivative liabilities	(15,200,000)		(15,200,000)		$ (3,500,000)
Unrealized mark-to-market (losses) gains	(17,000,000)	$ 1,800,000	(10,500,000)	(7,100,000)
Realized (losses) gains	(500,000)	(3,600,000)	(5,900,000)	11,200,000
Net derivative (losses) gains	$ (17,500,000)	$ (1,800,000)	$ (16,400,000)	$ 4,100,000