Derivatives (Cash Settlements) (Details) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ 472	$ (376)
Natural Gas Contracts
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	128	(198)
Interest Rate Swaps
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ 344	$ (178)