Fair Value Measurements (Narrative) (Details) $ in Thousands, lb in Millions	3 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($) derivative_instrument lb $ / lb	Jun. 30, 2019 lb $ / lb	Sep. 30, 2019 USD ($) derivative_instrument	Dec. 31, 2018 USD ($) derivative_instrument	Sep. 30, 2018 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			13.00%
Derivative, number of instruments held | derivative_instrument	6		6	5
Underlying, derivative mass | $ / lb	0.37	0.39
Derivative, nonmonetary notional amount, mass | lb	25.0	34.4
Derivative liability	$ 2,450		$ 2,450	$ 1,108	$ 2,500
Derivative liability, current				900
Derivative liability, noncurrent				$ 200