Derivatives Derivatives (Cash Settlements) (Details) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ (39)	$ (328)	$ (415)	$ (812)
Natural Gas Contracts
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	(36)	182	(234)	298
Interest Rate Swap
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ (3)	$ (510)	$ (181)	$ (1,110)