Restructuring - Net Pretax Benefits (Charges), Incurred by Segment (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) employee	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) employee
Restructuring Cost and Reserve [Line Items]
Approximate Number of Employees | employee		7		13
Restructuring charges	$ 0	$ 1,102	$ 0	$ 2,001
Changes in estimates		0		0
Total pretax charge, net of reversals		1,102		2,001
Operating Segments | Refined Coal
Restructuring Cost and Reserve [Line Items]
Restructuring charges		168		448
Changes in estimates		0		0
Total pretax charge, net of reversals		168		448
Operating Segments | Power Generation and Industrials
Restructuring Cost and Reserve [Line Items]
Restructuring charges		500		996
Changes in estimates		0		0
Total pretax charge, net of reversals		500		996
All Other and Corporate
Restructuring Cost and Reserve [Line Items]
Restructuring charges		434		557
Changes in estimates		0		0
Total pretax charge, net of reversals		$ 434		$ 557