Derivative Financial Instruments (Details) MMBTU in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) MMBTU	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) MMBTU	Sep. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) MMBTU
Derivative financial instruments
Unrealized net mark-to-market gains			$ 21,700,000	$ (11,600,000)
Open derivative contracts for natural gas (in MMBtus) | MMBTU	36.9		36.9			59.6
Percentage of natural gas consumption covered by derivatives			77.00%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position					$ 6,200,000	$ 27,900,000
Cash Collateral for Borrowed Securities	$ 0		$ 0		0	0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet					800,000	0
Gross amounts not offset in consolidated balance sheet
Financial instruments					800,000	0
Cash collateral received					0	0
Net amount					0	0
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet					(7,000,000)	(27,900,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments					(800,000)	0
Cash collateral pledged					0	0
Net amount					(6,200,000)	(27,900,000)
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet					$ (6,200,000)	(27,900,000)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Derivative Asset, Current	600,000		600,000			0
Derivative Asset, Noncurrent	200,000		200,000			0
Total derivative assets	800,000		800,000			0
Derivative Liability, Current	(3,600,000)		(3,600,000)			(15,900,000)
Derivative Liability, Noncurrent	(3,400,000)		(3,400,000)			(12,000,000)
Total derivative liabilities	7,000,000		7,000,000			27,900,000
Net derivative liabilities	(6,200,000)		(6,200,000)			$ (27,900,000)
Unrealized net mark-to-market gains	(3,300,000)	$ (17,000,000)	21,700,000	(10,500,000)
Realized net losses	(1,200,000)	(500,000)	(16,200,000)	(5,900,000)
Net derivative gains	$ (4,500,000)	$ (17,500,000)	$ 5,500,000	$ (16,400,000)