Derivatives Derivatives - Cash Settlements (Details) - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Additional Cash Paid on Settlement of Hedge	$ (1,883)	$ (4,589)	$ (4,567)
Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Additional Cash Paid on Settlement of Hedge	(47)	(2,345)	(4,567)
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Additional Cash Paid on Settlement of Hedge	$ (1,836)	$ (2,244)	$ 0