Organization and Significant Accounting Policies - Results of China Operations (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 15,827,000	$ 16,070,000	$ 11,847,000	$ 12,791,000	$ 13,386,000	$ 13,548,000	$ 9,741,000	$ 8,491,000	$ 56,535,000	$ 45,166,000	$ 55,161,000
Net loss	896,000	$ 1,045,000	$ (1,753,000)	$ (216,000)	(672,000)	$ (417,000)	$ (6,854,000)	$ (2,506,000)	(28,000)	(10,449,000)	(17,388,000)
Total assets	51,719,000				50,484,000				51,719,000	50,484,000
Total liabilities	17,667,000				16,312,000				17,667,000	16,312,000
Air Pollution Control Segment
Segment Reporting Information [Line Items]
Revenues									38,417,000	27,808,000	34,052,000
Beijing Fuel Tech | Air Pollution Control Segment
Segment Reporting Information [Line Items]
Revenues									3,006	8,034	5,087
Net loss									(1,868)	(1,343)	(2,300)
Total assets	8,546				13,005				8,546	13,005	13,474
Total liabilities	2,953				5,245				2,953	5,245	5,306
Total net assets	$ 5,593				$ 7,760				$ 5,593	$ 7,760	$ 8,168