Derivatives Derivatives - Cash Settlements (Details) - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ 585	$ (1,883)
Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	426	(47)
Interest Rate Swaps
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Additional Cash Settlements Received (Paid) on Hedge	$ 159	$ (1,836)