Derivative Financial Instruments (Details) MMBTU in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($) MMBTU	Dec. 31, 2014 USD ($)
Effect of Derivatives on Consolidated Statement of Operations
Unrealized net mark-to-market gains (losses)	$ 35,300,000	$ (24,300,000)	$ (11,400,000)
Open derivative contracts for natural gas (in MMBtus) | MMBTU	29.4	59.6
Percentage of natural gas consumption covered by derivatives	74.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 1,600,000	$ 27,900,000
Cash Collateral for Borrowed Securities	0	0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	9,000,000	0
Gross amounts not offset in consolidated balance sheet
Financial instruments	1,600,000	0
Cash collateral received (pledged)	0	0
Net amount	7,400,000	0
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	(1,600,000)	(27,900,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments	(1,600,000)	0
Cash collateral received (pledged)	0	0
Net amount	0	(27,900,000)
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	7,400,000	(27,900,000)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets	7,900,000	0
Derivative Asset, Noncurrent	1,100,000	0
Price Risk Derivative Instruments Not Designated as Hedging Instruments Asset, at Fair Value	9,000,000	0
Unrealized losses in other current liabilities	0	(15,900,000)
Unrealized losses in other liabilities	(1,600,000)	(12,000,000)
Price Risk Derivative Instruments Not Designated as Hedging Instruments Liability, at Fair Value	1,600,000	27,900,000
Net derivative assets (liabilities)	7,400,000	(27,900,000)
Effect of Derivatives on Consolidated Statement of Operations
Unrealized net mark-to-market gains (losses)	35,300,000	(23,100,000)	(12,600,000)
Realized net (losses) gains	(16,700,000)	(10,600,000)	10,500,000
Net derivative gains (losses)	$ 18,600,000	$ (33,700,000)	$ (2,100,000)