Segment Information - Net Sales by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Sales by Product
Net Sales	$ 137,480	$ 123,970	$ 132,597	$ 120,199	$ 130,873	$ 132,976	$ 135,452	$ 135,703	$ 514,246	$ 535,004	$ 555,103
Carbon products
Net Sales by Product
Net Sales									426,305	460,251	474,222
Capital equipment
Net Sales by Product
Net Sales									34,349	35,114	41,238
Equipment leasing
Net Sales by Product
Net Sales									14,041	15,235	16,598
Carbon cloth products
Net Sales by Product
Net Sales									8,695	9,163	11,564
Spare parts
Net Sales by Product
Net Sales									4,515	4,179	4,081
Other services
Net Sales by Product
Net Sales									14,219	$ 11,062	$ 7,400
Other products
Net Sales by Product
Net Sales									$ 12,122