Fair Value Measurements (Narrative) (Details) $ in Thousands, lb in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) lb $ / lb	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
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	14.00%
Combined carrying value	$ 304,144	$ 228,950	$ 210,304
Derivative, Nonmonetary Notional Amount, Mass | lb	3.0
Underlying, Derivative Mass | $ / lb	0.25
Derivative Asset	$ 700
Loss on derivative	$ 300