Derivatives Derivatives (Cash Settlements) (Details) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ (376)	$ (484)
Natural Gas Contracts
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	(198)	116
Interest Rate Swap
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ (178)	$ (600)