Derivatives Natural Gas Gain Loss Included in Other Income and Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative [Line Items]
Contracts where hedge accounting was not elected	$ (361)	$ (187)	$ (287)	$ (187)
Other Income (Expense) [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	566	0	167	0
Natural Gas Contracts | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Contracts where hedge accounting was not elected	58	0	(237)	0
Natural Gas Contracts | De-designated contracts [Member] | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	$ 508	$ 0	$ 404	$ 0