Derivative Financial Instruments (Details) MMBTU in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MMBTU	Dec. 31, 2014 USD ($) MMBTU	Dec. 31, 2013 USD ($)
Effect of Derivatives on Consolidated Statement of Operations
Unrealized mark-to-market (losses) gains	$ (24,300,000)	$ (11,400,000)	$ 8,600,000
Open derivative contracts for natural gas (in MMBtus) | MMBTU	59.6	7.6
Percentage of natural gas consumption covered by derivatives	64.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 27,900,000	$ 3,600,000
Cash Collateral for Borrowed Securities	0	0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	0	100,000
Gross amounts not offset in consolidated balance sheet
Financial instruments	0	100,000
Cash collateral received (pledged)	0	0
Net amount	0	0
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	(27,900,000)	(3,600,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments	0	(100,000)
Cash collateral received (pledged)	0	0
Net amount	(27,900,000)	(3,500,000)
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	(27,900,000)	(3,500,000)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets	0	100,000
Unrealized losses in other current liabilities	(15,900,000)	(3,600,000)
Unrealized losses in other liabilities	(12,000,000)	0
Net derivative liabilities	(27,900,000)	(3,500,000)
Effect of Derivatives on Consolidated Statement of Operations
Unrealized mark-to-market (losses) gains	(23,100,000)	(12,600,000)	9,100,000
Realized (losses) gains	(10,600,000)	10,500,000	(800,000)
Net derivative (losses) gains	$ (33,700,000)	$ (2,100,000)	$ 8,300,000