Derivatives Derivatives (Cash Settlements) (Details) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ 171	$ (505)	$ (244)	$ (1,317)
Natural Gas Contracts
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	48	(121)	(186)	177
Interest Rate Swaps
Derivative [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ 123	$ (384)	$ (58)	$ (1,494)