Derivative Financial Instruments (Narrative) (Details) $ in Thousands	1 Months Ended
	Jul. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) MMBTU / d $ / bbl Rate
Trading Activity, Gains and Losses, Net [Line Items]
Derivative, Fixed Interest Rate | Rate		1.81%
Notional amount of debt		$ 100,000
Diesel Swap Current Year [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU / d		(1,000)
Derivative, Swap Type, Fixed Price | $ / bbl		63.00
Subsequent Event [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Proceeds from Termination of Diesel Derivatives	$ 321