Derivative Financial Instruments (Details) MMBTU in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) MMBTU	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) MMBTU	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) MMBTU
Derivative financial instruments
Unrealized net mark-to-market gains (losses)			$ (9,700,000)	$ 21,700,000
Open derivative contracts for natural gas (in MMBtus) | MMBTU	13.7		13.7		29.4
Percentage of natural gas consumption covered by derivatives			49.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 2,600,000		$ 2,600,000		$ 1,600,000
Cash Collateral for Borrowed Securities	0		0		0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	200,000		200,000		9,000,000
Gross amounts not offset in consolidated balance sheet
Financial instruments	200,000		200,000		1,600,000
Cash collateral received	0		0		0
Net amount	0		0		7,400,000
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	(2,600,000)		(2,600,000)		(1,600,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments	(200,000)		(200,000)		(1,600,000)
Cash collateral pledged	0		0		0
Net amount	(2,400,000)		(2,400,000)		0
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	(2,400,000)		(2,400,000)		7,400,000
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Unrealized gains in other current assets	200,000		200,000		7,900,000
Unrealized gains in other assets	0		0		1,100,000
Total derivative assets	200,000		200,000		9,000,000
Unrealized losses in other current liabilities	(2,200,000)		(2,200,000)		0
Unrealized losses in other liabilities	(400,000)		(400,000)		(1,600,000)
Total derivative liabilities	(2,600,000)		(2,600,000)		(1,600,000)
Net derivative (liabilities) assets	(2,400,000)		(2,400,000)		$ 7,400,000
Unrealized net mark-to-market gains (losses)	1,100,000	$ (3,300,000)	(9,700,000)	21,700,000
Realized net losses	(1,400,000)	(1,200,000)	(1,400,000)	(16,200,000)
Net derivative (losses) gains	$ (300,000)	$ (4,500,000)	$ (11,100,000)	$ 5,500,000