Derivative Financial Instruments (Details) MMBTU in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MMBTU	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($)
Effect of Derivatives on Consolidated Statement of Operations
Unrealized net mark-to-market gains (losses)	$ (10,600,000)	$ 35,300,000	$ (24,300,000)
Open derivative contracts for natural gas (in MMBtus) | MMBTU	8.4	29.4
Percentage of natural gas consumption covered by derivatives	48.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 3,200,000	$ 1,600,000
Cash Collateral for Borrowed Securities	0	0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	0	9,000,000
Gross amounts not offset in consolidated balance sheet
Financial instruments	0	1,600,000
Cash collateral received (pledged)	0	0
Net amount	0	7,400,000
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	(3,200,000)	(1,600,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments	0	(1,600,000)
Cash collateral received (pledged)	0	0
Net amount	(3,200,000)	0
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	(3,200,000)	7,400,000
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets	0	7,900,000
Derivative Asset, Noncurrent	0	1,100,000
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	0	9,000,000
Unrealized losses in other current liabilities	(3,200,000)	0
Unrealized losses in other liabilities	0	(1,600,000)
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	(3,200,000)	(1,600,000)
Net derivative (liabilities) assets	(3,200,000)	7,400,000
Effect of Derivatives on Consolidated Statement of Operations
Unrealized net mark-to-market gains (losses)	(10,600,000)	35,300,000	(23,100,000)
Realized net losses	(3,000,000)	(16,700,000)	(10,600,000)
Net derivative (losses) gains	$ (13,600,000)	$ 18,600,000	$ (33,700,000)