Derivative Financial Instruments (Details) MMBTU in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) MMBTU	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) MMBTU	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) MMBTU
Derivative financial instruments
Unrealized net mark-to-market gains			$ 25,000,000	$ 5,600,000
Open derivative contracts for natural gas (in MMBtus) | MMBTU	44.4		44.4		59.6
Percentage of natural gas consumption covered by derivatives			79.40%
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position	$ 2,900,000		$ 2,900,000		$ 27,900,000
Cash Collateral for Borrowed Securities	0		0		0
Total derivative assets
Gross and net amounts presented in consolidated balance sheet	2,100,000		2,100,000		0
Gross amounts not offset in consolidated balance sheet
Financial instruments	2,100,000		2,100,000		0
Cash collateral received	0		0		0
Net amount	0		0		0
Total derivative liabilities
Gross and net amounts presented in consolidated balance sheet	(5,000,000)		(5,000,000)		(27,900,000)
Gross amounts not offset in consolidated balance sheet
Financial instruments	(2,100,000)		(2,100,000)		0
Cash collateral pledged	0		0		0
Net amount	(2,900,000)		(2,900,000)		(27,900,000)
Net assets (liabilities)
Gross and net amounts presented in consolidated balance sheet	(2,900,000)		(2,900,000)		(27,900,000)
Natural gas derivatives not designated as hedging instruments
Derivative financial instruments
Derivative Asset, Current	1,700,000		1,700,000		0
Derivative Asset, Noncurrent	400,000		400,000		0
Total derivative assets	2,100,000		2,100,000		0
Derivative Liability, Current	(1,800,000)		(1,800,000)		(15,900,000)
Derivative Liability, Noncurrent	(3,200,000)		(3,200,000)		(12,000,000)
Total derivative liabilities	5,000,000		5,000,000		27,900,000
Net derivative liabilities	(2,900,000)		(2,900,000)		$ (27,900,000)
Unrealized net mark-to-market gains	27,300,000	$ 2,600,000	25,000,000	6,500,000
Realized net losses	(7,600,000)	(800,000)	(15,000,000)	(5,400,000)
Net derivative gains	$ 19,700,000	$ 1,800,000	$ 10,000,000	$ 1,100,000