Fair Value Measurements (Narrative) (Details) $ in Thousands, lb in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) lb $ / lb	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
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	12.00%
Combined carrying value	$ 266,596	$ 228,950	$ 178,922
Derivative, Nonmonetary Notional Amount, Mass | lb	9.0
Underlying, Derivative Mass | $ / lb	0.25
Derivative Asset	$ 1,500
Derivative, Gain (Loss) on Derivative, Net	$ 500