Fair Value Measurements (Narrative) (Details) $ in Thousands, lb in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) lb $ / lb	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Fair Value Disclosures [Abstract]
Number of years long-term sales growth forecasted over	6 years
Average long-term sales growth forecast, over next 4 years, percent per year	11.00%
Combined carrying value	$ 282,142	$ 228,950	$ 203,204
Derivative, Nonmonetary Notional Amount, Mass | lb	6.0
Underlying, Derivative Mass | $ / lb	0.25
Derivative Asset	$ 1,300
Derivative, Gain (Loss) on Derivative, Net	$ 300