Derivatives Natural Gas Gain Loss Included in Other Income and Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative [Line Items]
Contracts where hedge accounting was not elected	$ 135	$ 461	$ (152)	$ 274
Other Income (Expense) [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	(42)	0	125	0
Natural Gas Contracts | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Contracts where hedge accounting was not elected	(222)	0	(459)	0
Natural Gas Contracts | De-designated contracts [Member] | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	$ 180	$ 0	$ 584	$ 0